Class Guide (OUTDATED)

This guide is meant to be a simple way of describing XCOM2's classes, their uses and some more options of gear and skills to choose.

Introduction

Hello and welcome to my Class Guide. A thing to take note of is that this is my first Steam Guide ever, so it's not gonna look very flashy to begin with. It's also a very early guide for XCOM2, so for all I know, it might get outdated at some point.

The general idea of this guide is to describe each of XCOM2's classes, their Rank Abilities, as well as an added personal view on gear and upgrade usage that perhaps may be of a little help to those new to XCOM in general.
Without any further delay, I'll get started on the actual guide now.


XCOM 2 has a total of 5 classes, 4 of which will be available to you when you start the game and one you unlock a while into the game.

The classes are the following: (Detailed descriptions are in the classes' own sections)

• Ranger - Frontline soldier and scout. Replaces the Assault class from XCOM:EU/EW.
  
• Grenadier - Heavy Weapons and Demolitions Expert. Replaces the Heavy class from XCOM:EU/EW.
  
• Specialist - Support, Medic and Tech Expert. Replaces the Support class from XCOM:EU/EW.
  
• Sharpshooter - Ranged sniper/assassin and Gunslinger. Replaces the Sniper class from XCOM:EU/EW.
  
• Psi Operative - Generally just an OP Mind***er. Unlocked after building the Psi Chamber. Replaces the Psionic sub-class from XCOM:EU/EW, becoming an entirely new Class.

Another Class is also being added in the Shen's Last Gift DLC (Possibly a replacement of the MEC Trooper from EW), which will be released later in 2016.


Notes:

• All Build Examples are made from my own experience with the game and are the builds I actively use myself. If there are skills, items, etc you personally don't like using, I suggest experimenting with things on your own to find what you like.
  
• I couldn't find an image of the Sergeant rank, or any of the Psi Op. ranks, so those just have names...
  
• Take special note of that my Build Examples have a 3rd Weapon Upgrade listed. This is because of the Armed to the Teeth Continent Bonus you may or may not get while playing (it's random wether or not this Bonus actually is obtainable in a playthrough and which continent it is available on), which gives all your Weapons an extra Weapon Upgrade Slot.
  
• One last thing I want to mention is that my Build Examples does NOT consider skills you might get from having the Advanced Warfare Center. Since most of these skills will be useful to you one way or another anyway, I recommend experimenting with skills that you think will fit better with them.

If you miss something in this guide, feel free to write a comment about it, and I'll consider adding it.


To-Do-List:
Rewrite the whole thing
Add Alien-Hunter DLC stuff

The Grenadier

"Serving as our demolitions experts, the Grenadiers provide heavy ordnance delivery whenever and wherever we need it."
-Central Officer Bradford about the Grenadier

Being your Squad's Demolitions and Heavy Weapons Experts, the Grenadier is useful for both an offensive and supportive role. Easily able to take down enemy cover, shred armor and cause high area damage with their explosives and Cannon (actually more like a Mini-Gun), they are one of the most important classes mid to late-game. Having one or two in your squad doesn't hurt when the enemy starts sending in their heavy armor.

Skills

The Grenadier's two skill-sets both focus on heavy offensive abilities, but also some supportive ones. The Demolitions Expert focuses on the usage of Grenades and Heavy Weapons, while the Heavy Gunner focuses more on using the Grenadier's Main Weapon, the Cannon.

Launch Grenade	The Grenadier uses a grenade launcher to fire grenades rather than throwing them. This allows for greater grenade range. The Grenadier gains an extra inventory slot that may only be used for grenades.
Blast Padding	The Grenadier's gear includes layers of extra padding and blast plates, granting a bonus point of Armor and 66% less damage from explosive attacks.		The Grenadier's Cannon attacks shred armor. Higher rank weapons shred more points of armor. The shredding effect applies before damage.	Shredder
Demolition	Activated. The Grenadier unleashes a volley of bullets at the target's cover, significantly damaging or destroying it, but deals no damage to the target. Uses 2 ammo. 3-turn cooldown.	Sergeant	Activated. Fires a barrage that pins down the Grenadier's target for one turn, granting Reaction Fire against it if it moves, and imposing a -50 penalty to the target's Aim. Suppression is cancelled if the Grenadier takes damage. Suppression penalties stack. Uses 2 ammo. No cooldown.	Suppression
Heavy Ordnance	The grenade in the Grenadier's grenade-only slot gains a bonus use. Affects support grenades as well as offensive ones.		Any directed cannon shot, hit or miss, will mark the Grenadier's target, increasing the Squad's Aim by +15 against this target. Applies to standard attacks and single-target abilities, including Suppression.	Holo Targeting
Volatile Mix	Grenades gain +1 tile to their radius and deal +2 damage. Does not increase environment damage.		Activated. Fires a shot with a penalty of 15 to aim. If it hits, the Grenadier immediately fires another shot at the target. Uses 2 ammo. 3-turn cooldown.	Chain Shot
Salvo	Using the Grenade Launcher or a Heavy Weapon as the Grenadier's first action does not end the turn.		Activated. Fire a shot that is guaranteed to hit. Uses 3 ammo. 5 turn cool down.	Hail of Bullets
Saturation Fire	Activated. Fires a hail of bullets in a cone damaging every enemy and all cover within. Uses 3 ammo, 5 turn cool down.		Activated. Fires a shot that deals critical damage. The target, if hit, will also take 3 additional damage from all sources for the rest of the mission. Uses 3 ammo. 3-turn cooldown.	Rupture

The Grenadier - Build Examples

This is the build I personally have used for my Grenadier so far with extremely good results. Note that this build is meant for late-game, so you might have to improvise until you get some of the items mentioned, but I will add notes that may help a bit while getting them.

Anti-Armor

This build focuses on the supportive role of the Grenadier, as well as being very efficient on the offensive part. I focus my Grenadiers' fire on anything wearing armor before attacking with the rest of my squad, simply to get the most out of the other attacks. I also use it to blow up buildings for easier manuverability of the terrain and make my Sharpshooters able to shoot "through" buildings.

Items
Armor: E.X.O./W.A.R.(+Armor)
Heavy Weapon: Shredder Gun/Shredstorm Cannon
Weapon Upgrades: Scope (+Aim), Extended Magazine (+Ammo), Stock (Missed Damage)
Utility Item: Frag/Plasma Grenade or AP Rounds
Grenade: Frag/Plasma Grenade
PCS: Perception (+Aim) or Conditioning (+HP)

Notes:

• Once you get it, the Shredder Gun's/Shredstorm Cannon's ability to tear down a massive amount of cover is extremely helpful in most situations it can be used, as well as shredding armor and dealing a lot of damage to enemies.
  
• Switching out the Shredder Gun/Shredstorm Cannon with the Rocket/Blaster Launcher might be a good addition to the amount of explosives you already have, though you will lose a lot of damage if you already have Shredstorm Cannon (if you blow up lots of walls like I do, the Rocket Launcher is better).
  
• Being the Class with the worth Aim stat, having a Scope upgrade is important. The Stock upgrade is also extremely useful, as the Grenadier will likely miss a lot of shots.
  
• Early to mid-game, having a Frag Grenade as the Grenadier's Utility Item is the best choice, as you will be able to use 2 (or 3, if you have the Heavy Ordance ability) Grenades. I personally mostly use Grenades to tear down walls so my Sharpshooter can shoot inside or through buildings.
  
• Mid to late-game, having AP Rounds will let you deal full damage to armored units as well as tearing off their armor with the Shredder ability.
  
• Switching the Frag in the Grenade Slot with a Proximity Mine can be an option to help controlling the enemy's movements (they do try avoiding the mines if they are aware of the user when they are thrown) or to enter combat with a (sneaky) blast. However, having an extra Grenade (for the reasons I mentioned above) might also be more useful.
  
• You may also consider switching out the Grenade slot with a Incendiary (Sets pretty much everything on fire), Gas (Posions organic units) or Acid (Removes armor) Grenade/Bomb.
  
• As mentioned before, the Grenadier is the class with the lowest average Aim stat. You can boost this with the Perception (+Aim) PCS to make sure more attacks hit. Use the Conditioning (+HP) PCS if you would rather want to focus on defense.

Skills
Shredder - Essential for your Squad's survival mid to late-game. Depending on their Cannon's level, Shredder makes the Grenadier able to remove an enemy's armor very efficiently, as long as they hit. This helps gaining an upper hand very easily, specially when fighting late-game enemies like the Sectopod and Gatekeeper.

Suppression - One of the best supportive abilities that the Grenadier can get, Suppression lets you trap an enemy for one turn, taking a shot at them if they move and giving them an Aim penalty if they try attacking. Best to use as a last-of-the-turn action if your shot % is low, so one of you other units can take it down on your next turn (unless it moves, gets shot and dies).

Holo Targeting - Another good supportive ability, giving the Squad a +15 Aim bonus when attacking the Grenadier's target whenever they use their Cannon (Also counts when you use Suppression). This bonus is shown as a marker on the target, in case you happen to forget who you were shooting at. Also, its applied even if the Grenadier misses the shot, so it can still be helpful!

Chain Shot - Similar to the Ranger's Rapid Fire, Chain Shot makes the Grenadier shoot twice in a row. However, only if the first shot hits. With the added bonuses from the Scope upgrade and the Perception PCS, the Aim penalty from Chain Shot is also reduced, making it easier for those two shots to happen. Deals massive damage, applies Holo Targeting and Shredder if successful.

Salvo - Having Salvo lets the Grenadier fire off a Grenade or their Heavy Weapon at the start of their turn and still get to move or shoot their Cannon. Very useful, and you don't waste 3 ammo at a guaranteed shot on Hail of Bullets. (That's what I have those Aim bonuses for)

Rupture - An attack that shreds armor and applies an effect to the target that makes it take 3 more damage from all sources for the rest of its short life. Also applies the effects of Holo Targeting and Shredder (so most likely the poor thing won't have more armor after being hit with this).

The Ranger

"The Ranger serves as our primary reconnaissance unit, capable of moving independently in concealment while engaging enemies at close range."
-Central Officer Bradford about the Ranger

A front-line Solider, the Ranger is usually the first to enter combat and the last to get out. Wielding a Sword and a Shotgun or Assault Rifle, the Ranger can choose between standing face-to-face with and taking the attention of the enemy or scouting ahead of the Squad by staying out of enemy sight and moving far at once. Probably one of, if not the most important Class to have in your squad, choosing the right Skills and Equipment for the Ranger can be essential for a mission's success.

Skills

The Ranger has two Skill-sets: Scout and Blademaster. As their names hint to, one focuses on being able to stay out of the enemy's sight, scouting ahead of the rest of the Squad and flanking and taking the enemy by surprise, while the other focuses on close-combat and avoiding enemy fire.
Many of the Ranger's skills are Passive, meaning they are always in effect or have certain conditions to trigger, while only a few needs to be activated.

Slash	Activated. The Ranger attacks an enemy within movement range with their Sword. No cooldown.
Phantom	When the squad is revealed, the Ranger remains Concealed.		The Ranger deals +2 extra damage on all Sword attacks.	Blademaster
Shadowstrike	While Concealed, the Ranger gains +25 bonus Aim and +25 bonus Critical Chance when attacking enemies.	Sergeant	The Ranger does not trigger Overwatch or Reaction Fire.	Shadowstep
Conceal	Activated. The Ranger immediately enters Concealment. One use per mission.		Activated. The Ranger can take an action after Dashing. 3 turn cooldown.	Run and Gun
Implacable	If they score one or more kills on your turn, the Ranger is granted a single bonus move.		Free Sword attacks on enemies that enter the Ranger's melee range or attacks the Ranger from melee range.	Bladestorm
Deep Cover	If the Ranger did not attack this turn, they uses Hunker Down automatically.		If the Ranger score a kill during their turn, the next attack against them during the enemy turn will miss.	Untouchable
Rapid Fire	Activated. The Ranger fires twice in a row at an enemy. Each shot suffers an Aim penalty of -15. No cooldown.		A devastating chain melee attack where the first melee attack cannot miss. Each melee kill in Reaper mode grants the Ranger an extra action, but further melee attacks deal reduced damage. 4 turn cooldown.	Reaper

The Ranger - Build Examples

These are the builds I personally have used for my Rangers so far with pretty good results. Note that these builds are meant for late-game, so you might have to improvise until you get some of the items mentioned, but I will add notes that may help a bit while getting them.

Close-Combat Expert

This Close-Combat Ranger is the build I personally like the most. Able to soak up a lot of damage as well as taking the enemies' attention off of the rest of the Squad once it gets into combat, as well as being a very efficient killer, its useable in pretty much every situation I've been in so far. Sneaking around the enemy before starting an excessive amount of crit hits is actually pretty fun.


Main Weapon: Shotgun
Armor: E.X.O./W.A.R. (+Armor)
Heavy Weapon: Shredder Gun/Shredstorm Cannon
Utility Item: Plated Vest (+2 Armor) or Stasis Vest (Heals)
Weapon Upgrades: Laser Sight (+Crit Chance), Stock (Missed Damage), Repeater (Instant Kill)
PCS: Conditioning (+HP)

Notes:

• The Shotgun + Laser Sight combo is essential, as you get up to 35% Crit Chance, rising the chance to easily kill off targets that are getting a bit too comfy up close.
  
• Only having a single charge, using the Shredder Gun/Shredstorm Cannon at the right time is important. Try positioning the Ranger to catch as many (armored) enemies within the cone of fire. Also effctive at taking down enemy cover for your other units to get an easier shot.
  
• You can eventually replace the Shredder Gun/Shredstorm Cannon with a Flamethrower/Hellfire Projector to put things on fire (Note that this is not as efficient against robotic enemies and won't remove Armor like the other option does) or a Blaster Launcher, which works like a guided Rocket Launcher with much more damage.
  
• Replacing the Repeater upgrade with a Hair Trigger (Free Action) is an option, but the Instant Kill chance is very effective against most late-game enemies, as it ignores both Armor and high HP when triggered.
  
• Giving the Ranger a wooping 4 Armor at most (that's the same as a Gatekeeper in hiding), the Heavy Armor + Plated Vest combo makes the Close-Combat Ranger extremely resilent, and easily outweights the Stasis Vest in efficieny.
  
• Replacing the PCS with Speed (+Mobility) or Agility (+Dodge) is an option to be able to move further or escape some damage, as the HP bonuses and Armor it already has helps keeping the Ranger alive for a while.

Skills
Phantom - Having Phantom will make it easier for the Ranger to get closer to the action if you accidentally get spotted with another Squadmate. In my opinion outweights Blademaster in most situations.

Shadowstep - A must for the Close Combat Ranger, as using Slash, shooting, etc. triggering Overwatch and Reaction Fire is something you want to avoid as much as possible, preferably without wasting your other Squadmates' actions to get close to or kill an enemy. Can also be used to remove Overwatch so your other units can move freely.

Run and Gun - As the Close Combat Ranger is using a Shotgun, being close to their target is important. Run and Gun lets them dash as close as they can, then get a high % shot at an enemy.

Bladestorm - Being close to the enemies means one thing: The Ranger WILL be surrounded at times. Bladestorm lets them freely attack enemies that gets into melee range (adjacent tiles), as well as any enemies attempting to attack them or others while next to the Ranger. As the effect triggers BEFORE an enemy can attack, having Bladestorm might just save one of your units from a wound or worse.

Untouchable - Your Ranger will most likely be one of your most effective killers, and being able to avoid attacks is a good defensive ability. Why not hit two birds with one stone (You might actually be able to do that, kind of...) and add both those things together?

Rapid Fire - While the Reaper ability is certainly good against enemies with low HP, it is pretty much useless when you might wish it didn't only trigger on kills. Especially late-game, where armored enemies will negate most of the damage from Swords. Having Rapid Fire instead lets you shoot twice, then ask questions like "Why would I go Melee? lol" later, likely to a dead alien.

Scout

The Scout Ranger focuses on mobility and stealth rather than firepower and HP, Mainly meant to give the Sharpshooter more targets, I use this build rarely, as they usually have enough targets while the rest of my Squad is busy being beaten around somewhere ahead. But if you want a sneaky ninja-like Ranger that stays in the shadows and moves around a lot, this is probably a good build.

Items
Main Weapon: Rifle
Armor: Spider/Wraith (+Mobility)
Utility Item: Proximity Mine
Weapon Upgrades: Hair Trigger (Free Action), Laser Sight (+Crit Hit), Stock (+Aim)
PCS: Speed (+Mobility)

Notes

• Alternately, using Predator/Warden armor for the extra Utility Item slot can be useful, but in my opinion not necessary for the Ranger unless you want another Mine, Grenade or Bomb for some area-damage.
  
• Mid to late-game you might want to replace the Proximity Mine with AP Rounds or an Acid Grenade/Bomb to be able to ignore or shred armor from enemies. Note that unlike using a Proximity Mine, an Acid Grenade/Bomb will break concealmeant.
  
• An Overdrive Serum can replace the Proximity Mine, giving increased mobility. However, since you consume the item when its used, I recommend avoiding it, as Berserker Corpses honestly isn't something you will have loads of.
  
• Replacing the Laser Sight upgrade with a Scope can be helpful if you find your Ranger missing a lot of shots.

Skills
Phantom - Having Phantom will let the Ranger stay Concealed even if the rest of your Squad enters combat. A must-have for this build.

Shadowstep - Being able to avoid Overwatch and Reaction Fire is great when you want to avoid taking a lot of damage. While Shadowstrike can also be useful, I personally prefer Shadowstep due to its defensive usage when moving around.

Run and Gun - This will let you move away from enemies, then fire or enter Overwatch. Only use when the Ranger's initial Concealment is broken, else its just a waste. Could be replaced with Conceal, but in my opinion, once you've broken Concealment once, you can't really put up those Ambushes as effectively anyway... You also need to stay out of the enemy's sight for your Concealment not to be broken again immediatelly.

Implacable - Let's the Ranger move if they kill an enemy. Kind of a reverse Run and Gun without a cooldown and very useful to keep the Ranger away from enemies.

Untouchable - While your Scout Ranger does not have a focus on killing, it is inevitable that it will happen at some point, as not being permanently Concealed will draw some unwanted attention from those ADVENT and Aliens. While being able to Hunker Down automatically after just moving, being able to defend themselves is a thing even the Ranger should be allowed to do. Whenever they DO kill an enemy, the first attack against them will miss, no matter what. Which is nice, specially since your Scout will be a bit squishy compared to the rest of you Squad.

Rapid Fire - Very useful late-game, as you get to fire 2 shots at once, making it easier to kill most of the dangerous high HP enemies. Also gives you a 2x chance at a Free Action from the Hair Trigger upgrade.

The Sharpshooter

"Just like it sounds, our sharpshooters engage enemy targets with pinpoint accuracy from extreme range. They're also trained in pistol marksmanship for the occasional close encounter."
-Central Officer Bradford about the Sharpshooter

Usually staying far behind the rest of your Squad, the Sharpshooter gives your units cover fire from (preferably) somewhere the enemy can't get to them. Being extremely deadly from long ranges, the Sharpshooter serves as your assassination unit, able to deal a load of damage from the other side of the map if possible.

Skills

The Sharpshooter's skill sets, Sniper and Gunslinger, focuses on Long Range and Close Range respectively. While it might be tempting to focus only on one of these sets, doing a bit of mixing might also be useful, just in case your Sharpshooter happens to get up close to an enemy.
The only Class able to use a Pistol, the Sharpshooter is able to fend of enemies with low HP by themselves if they happen to get ambused, but as likely as this is, the Pistol skills is more useful to finish off low HP enemies that are in it's limited range.

Pistol	The Sharpshooter has a second weapon, a Pistol, that they can fire after moving, but with a shorter range and less damage. Also allows the Sharpshooter to use Overwatch after moving within the Pistol's range.		You can target enemies within squadmates' sight, provided there is line of sight to the target.	Squadsight
Long Watch	Allow Overwatch to trigger with Squadsight.		When targeted by enemy fire, automatically fire back with your pistol once per turn.	Return Fire
Deadeye	Take a shot with a small aim penalty for a significant damage boost. 2 turn cooldown.	Sergeant	Fire your pistol at a target. This attack does not cost an action. 3 turn cooldown.	Lightning Strike
Death From Above	Killing an enemy at a lower elevation with your sniper rifle costs only a single action and does not end your turn.		Firing your pistol with your first action no longer ends your turn.	Quickdraw
Kill Zone	Take a reaction shot against any enemy that moves or attacks within a cone of fire. 3 turn cooldown.		Fire once at every visible enemy with your pistol. 3 turn cooldown.	Faceoff
Steady Hands	If you didn't move last turn, gain +10 aim and +10 critical chance.		Hunker Down now confers +20 aim to the first shot on the following turn.	Aim
Serial	Each kill made with your sniper rifle completely restores your actions. With each consecutive kill you get a critical chance penalty. 4 turn cooldown.		Fire the pistol 3 consecutive times at a single target. 3 turn cooldown.	Fan Fire

The Sharpshooter - Build Examples

This is the build I personally have used for my Sharpshooters so far with good results. Note that this build is meant for late-game, so you might have to improvise until you get some of the items mentioned, but I will add notes that may help a bit while getting them.

Sniper

This build is pretty straight-forward, focusing on the Sharpshooter stay behind at a sniper-nest and wounding or killing anything they can get their sights on. When using this build, your first focus on a mission will be to get the Sharpshooter to a high location near the LZ where it can stay for most of the mission's duration.

Armor: Spider/Wraith (+Mobility)
Utility Item: AP Rounds
Weapon Upgrades: Auto-Loader (Free Reloads), Extended Magazine (+Ammo), Hair Trigger (Free Action)
PCS: Precision (+Aim)

Notes:

• Using any other Armor but the Light version is not recommended, as the Grappling Hook will let the Sharpshooter get to the highest places on most maps. The Grappling Hook also doesn't spend an action and will let the Sharpshooter use their Sniper Rifle directly after using it.
  
• Early to mid-game, it might be a good option to switch out the AP Rounds with Venom Rounds, as you won't be meeting a lot of Robotic or Armored enemies until later.
  
• The Auto-Loader and Extended Magazine upgrade combo is soely used for the purpose of making as much as possible out of the Serial ability when you get to use it (with Superior upgrades you will be able to shoot up to 20 times with it...), but will also will let you move further after reloading, if necessary, and being able to shoot more before reloading in general.
  
• Switching out the Hair Trigger with a Scope might be a good option if you have trouble hitting enemies at extremely long range, but that will not be necessary in most cases where the Sharpshooter has a clear shot.
  
• Already having a high Aim stat, the PCS may be switched out with Speed (+Mobility) to be able to move further when needed.

Skills
Long Watch - Let's the Sharpshooter's Overwatch trigger from Squadsight, meaning it can trigger as long as a friendly unit can see the enemy moving. This is very useful in pretty much every situation you get into.

Lightning Hands - Let's the Sharpshooter fire their Pistol at a nearby enemy, but not spending an action. Useful to finish off enemies with low HP, then firing the Sniper Rifle (or Pistol) at another enemy.

Death From Above - As your Sharpshooter will be spending most of it's time at a high position, Death From Above is a good skill choice. If you kill an enemy while at a high altitude than them with the Sniper Rifle, the turn won't end, and the Sharpshooter can move or fire their Pistol.

Faceoff - While not having as long range as Kill Zone, I've found Faceoff to be more useful in a pinch. Using it makes the Sharpshooter fire their Pistol at every visible enemy within range, and might actually save your squad in some situations. (Note that they will also fire at enemy VIPs, Mind-Controlled allies and Mind-Controlled or Hacked enemies).

Steady Hands - Unlike Aim, Steady Hands doesn't involve you wasting a turn on Hunker Down. As long as the Sharpshooter doesn't move, even if it uses any other actions, Steady Hands' bonuses will be added to the Sharpshooter's attack on the next turn. It does not stack if you don't move on consecutive turns, but it does stay as long as you stay in the same spot.

Serial - As I mentioned in the item notes, Serial can let the Sharpshooter kill a maximum of 20 enemies in one turn with the right Weapon Upgrades. When you activate Serial, every kill you make with you Sniper Rifle restores your actions. This means that as long as you're able to kill an enemy, you will be able to do another shot. And you don't even have to worry about reloading, as you can shoot after, and if you kill another enemy you get BOTH actions back. If you somehow accidentally manage to choose Fan Fire over Serial... well

The Specialist

"Operating some of our most advanced equipment, Specialists deploy robotic drones on the battlefield that can be outfitted for combat or field medic duty."
-Central Officer Bradford about the Specialist

The Support class from XCOM: EU/EW got a substainsial upgrade in XCOM2 in the form of the GREMLIN Drone, able to heal their Squadmates at distance, protecting them and (probably the best) being able to heal and/or remove negative effects from everyone at once. They are also able to use their flying friend to hack enemy robots, turrets, sentries, doors and computers. Their little friend somehow remains invisible to the enemy at all times and only gets destroyed if their owner happens to die. This makes the Specialist class extremely useful in more than one way.

Skills

The Specialist's abilities depend a lot on their GREMLIN Drone, almost every Rank Skill giving it some sort of upgrade. They can also choose to focus on medical support as a Battle Medic or somewhat more offensive and technical support as a Combat Hacker.
When on a timed mission where you need to hack a ADVENT Network access point, the Hack ability can be used from a distance, as long as the Specialist has the target in sight and is within a certain range. This is very useful if you suddenly happen to run out of time fast.

Hack	The Specialist attempts to breach security on a network access point with the GREMLIN.		Commands the GREMLIN to move to a friendly target. It grants that target a bonus to Defense until the start of the next player turn.	Aid Protocol
Medical Protocol	The GREMLIN can perform healing actions remotely: GREMLIN Heal and GREMLIN Stabilize. The GREMLIN has a single charge. If a Medkit is equipped, the GREMLIN will gain an additional charge (2 additional charges with the FIeld Medic skill).		Send the GREMLIN to an enemy to jolt them, dealing guaranteed damage, which is increased against robotic enemies. Two uses per mission.	Combat Protocol
Revival Protocol	Send the GREMLIN to an ally to remove any negative mental status effects (Disoriented, Stunned, Panicked, or Unconscious).	Sergeant	The Specialist may target robotic and mechanical enemies with their GREMLIN, attempting to hack them and seize control.	Haywire Protocol
Field Medic	Equipped medikits have 2 extra charges.		The GREMLIN can trigger an instant scan of the area, increasing the Specialist's sight radius substantially for one turn and revealing any hidden enemies.	Scanning Protocol
Covering Fire	Overwatch shots can now be triggered by enemy actions, not just movement..		Aid Protocol now grants the target a Covering Fire Overwatch shot, but the Aid Protocol cooldown is increased by 1 turn.	Threat Assesment
Ever Vigilant	If the Specialist spend all their actions on moves, they are granted an automatic overwatch shot at the end of the turn.		Grants 50% chance during Overwatch to perform an additional shot if the previous Overwatch shot hits. Can be triggered several times on the same turn.	Guardian
Restoration	The GREMLIN flies to each squad member, healing or reviving them as needed. One use per mission.		Sends the GREMLIN to a location where it emits a substantial electric discharge, damaging and potentially stunning all nearby units. Robotic units take more damage. One use per mission.	Capacitor Discharge

The Specialist - Build Examples

This is the build I personally have used for my Specialists so far with pretty great results. Note that this build is meant for late-game, so you might have to improvise until you get some of the items mentioned, but I will add notes that may help a bit while getting them.

Tech Medic

The Tech Medic is mainly focused on giving your other units Medical support, but also has a few abilities that is useful for other things. Having the Haywire Protocol ability, they can hack into enemy robots and turrets, either shutting them down temporarily or taking over their system so you can use them as an additional Squad unit and a distraction.

Items
Armor: Predator/Warden (+Utility Item)
Utility Item: (Nano)Medikit and AP Rounds
Weapon Upgrades: Hair Trigger (Free Action), Stock (Missed Damage), Repeater (Instant Kills)
PCS: Focus (+Will)

Notes

• Carrying a Medikit is probably the most important thing the Specialist does, as long as they have the Medical Protocol skill. Make sure to always have one with your Specialist.
  
• Early to mid-game, the AP Rounds should be replaced with other types of Ammo, as you won't be meeting a lot of armored enemies until later. I recommend Venom Rounds, as you won't be meeting a lot of Robotic enemies either.
  
• The Focus PCS can be replaced with Conditioning (+HP) or Speed (+Mobility) for better surival. I only use the Focus PCS to make sure my only medics does not get Panicked or Mind-Controlled.

Skills
Medical Protocol - Lets the GREMLIN Drone heal a friendly unit. Unlike every other Class (includes the Support from EU/EW), the Specialist does not need to be close to the unit being healed, as the GREMLIN will simply fly over and heal it from mostly anywhere on the map. Amount healed depends on the GREMLIN's level and what type of Medikit you are carrying. Charges can be increased with skills (Field Medic).

Haywire Protocol - Lets the GREMLIN Drone hack an enemy Robotic unit, either stunning the unit or temporarily let you control it. This works like any other Hack in the game, and you have a chance to fail, giving a feedback result (usually gives the unit higher defense and an Aim boost for a few turns).

Field Medic - If the Specialist is carrying a Medikit, it has 3 charges instead of 1. This affects the charges of Medical Protocol.

Threat Assessment - The GREMLIN's Aid Protocol now also makes the target enter Overwatch. I personally tend to use this on my Rangers, as Aid Protocol already gives a Defense bonus, and having a Ranger close to the action enter Overwatch is almost always a good thing.

Ever Vigilant - A very useful supportive and offensive skill that makes the Specialist automatically enter Overwatch if all of their actions were spent moving. Spending their actions to get behind cover, then being able to defend against potential flanking enemies is a great addition to the Specialist's many supportive abilities.

Restoration - Makes the GREMLIN fly over to any wounded, fainted, poisoned, burned, etc. allies unit and heals them of their condition. As Restoration has its own charge, it does not depend on you carrying Medikits or having any left, for that matter. Note that if you happen to be Mind-Controling or Hacking an enemy, they will count as friendly units and the GREMLIN will heal them as well, but does not break the Mind-Control/Hack.

The Psi Operative

"Depending on who ou ask, Psionics could be considered the aliens' greates strength. Now our own Psi Operatives can tap into that same power."
-Central Officer Bradford about the Psi Operative

Unlike the Psi Operatives in EU/EW, XCOM2's Psi Ops are not a Sub Class, and must be trained by a Rookie Soldier like any other Class. However, they can only learn to become a Psi Operative through training at the Psi Chamber, which you will unlock a bit into the game campaign.
A largely supportive, yet extremely dangerous Class, the Psi Operative uses the Aliens' own psionic powers against them. Using the vast powers of their trained minds, they boost and inspire their allies while controling, laying waste to and bring fear to their enemies, preferably all at once.

Skills

One specific thing that differentiates the Psi Operatives from the other classes of XCOM 2, is that they do not gain Skills through going on missions. Like they become Psi Operatives, they must train at the Psi Chamber to obtain new skills. However, unlike the other classes, they don't have to choose from a limited set of skills, but can rather train ALL the abilities available to them, and does also not obtain them through the same rank-system as the others, but rather randomly while training. It might take some time getting all of them, but eventually having a living fortress of destruction is worth it.

Soulfire	Does guaranteed psionic damage to an organic enemy, ignores cover and armor.	Initiate	Completely stuns the target for 1 turn, but renders them immune to any damage or attack.	Stasis
Insanity	Debilitating telepathic attack that can inflict different negative conditions, including mental control of the target.	Acolyte	Grant a bonus action immediately to a nearby squadmate.	Inspire
Soul Steal	Using Soulfire now heals you for half of the damage dealt	Adept	Stasis can be used on friendly units	Stasis Shield
Solace	The Psi Operative is surrounded by an aura that immediately extinguishes or blocks any mental impairments for themselves and any nearby allies. Passive.	Disciple	Once per mission, when the psionic soldier is reduced to 0 hit points, he is instead reduced to 1 hit point and put into stasis for 1 turn.	Sustain
Schism	Insanity now does a small amount of guaranteed damage, and applies Rupture to the target.	Mystic	The Psi Operative is immune to fire, poison, acid, and explosive damage.	Fortress
Fuse	If an enemy is carrying explosives, they can be remotely detonated by the Psi Operative.	Warlock	Permanently mind controls an enemy. Only one successful Domination can be performed per mission.	Domination
Null Lance	Projects a beam of psionic energy that damages every target it passes through.	Magus	A large Area of Effect ability dealing moderate damage and a chance to inflict insanity on all targets in the storm.	Void Rift

Unlike the other Classes, making builds for the Psi Operatives is not really that hard, as all you have to do is choose the items that you feel best fits to your useage of them, so I won't be giving any Build Examples for them.
EDIT: Actually, after some thought, I might be adding a build example after all, depending on wether or not there is any interest in this by people reading and using this guide. I'll wait a few weeks and see if there is any responses to this though.

[Arretrea]

Oh wow, this guide has actually taken off a bit. Last time I touched this was back in February last year, and I don't think I've played the game since then either ^^'
Thanks for all the new favorites and such. Great to see it being useful to people :D

Despite it being outdated and such, I'll at least try to make it a bit easier to read. I see that specially the build examples can be really hard on the eyes... like, literally walls of text. Fixing that as soon I have time ^^

[Arretrea]

As for modding, I have no experience whatsoever. I know there is a lot of Custom Class mods, some of which I've tried out myself. My suggestion to your question would be to look into some of them, maybe leave a similar question with those guys, or google how to make a custom class mod. I don't know whether or not you'll find anything useful, but it would at least be worth a try to check ^^

[Arretrea]

My assumption the Domination skill is a 1-use simply because of how overpowered it is. Being able to permanently control an enemy unit, when used on the right unit, can cause a fight to completely tip the scales after all. And with most enemies being controllable (everything that isn't a machine, but that's easy to solve using a Specialist's hacking ability instead), it's easy to see why they decided to go with just 1 use. Who knows, there might be a mod out there for it to have more uses (or someone willing to make one).
I don't remember exactly, but I also think there might be a cheat code that lets you recharge abilities, which probably can be used for it. I don't remember what that would be though :/ Heck, I don't even remember how to enter cheat codes at this point.

[JCGemX]

also do you know by any chance if there is a way to make a class, that can use any of the other

classes abilities/skills and weapons? I've was hoping there was a "Custom Class Creator" mod

or something on the workshop, that I could use to make my own class without needing any

experience in modding?

[JCGemX]

is there a way to modify the "Domination" skill so it doesn't have the "Only one successful Domination can be performed per mission." enabled?

[Arretrea]

Yes. You get them randomly, but it's a very small chance that you'll be getting the last tier abilities first.
I have had newly trained Initiates with Void Rift, so I can confirm that it's possible.

[Chett Wong]

When i train a Psi op can i get the last skill on the picture first?

[Arretrea]

Nope, sadly :/
I have a character that I "brought over" that's supposed to be the first Psi I got in EU (made in the character creator, based on the soldier), but since I keep getting him in the first mission, he gets other classes all the time, so I can't make him a Psi unless I *cough* hack *cough* or use mods.

[Chett Wong]

So i can't have my sniper upgraded to Psi? Shame :( ty for replying<3

[Arretrea]

You have to train them as Psi Ops from Rookie level. So kinda like in EU/EW, except you can't train a soldier that already has another class.