Left 4 Dead 2 is an amazing game and your interest in this guide shows you are willing to learn the basics and how to play the game. Just know that this is a difficult game and lack of teamwork will result in you and your team dying.
The Goal Of The Game
Left For Dead 2 has numerous modes each with their own goals. Most gamemodes are based off of basic campaigns however. In a campaign, four players are tasked with making it to the safehouse at the end of each map. The journey is perilous and various groups of infected will try to stop you in the only way they know how, killing you in their own unique way. Each campaign usually ends with a finalie level where a MASSIVE group of zombies tries to wipe out the survivors. When one survivor dies, they can be revived with defibulators or be found in a respawn room somewhere on the map. If the whole group dies, the map restarts.
Mutation: Mutation is a strange gamemode with different objectives and game modifications. Some are simple such as zombies being stronger or even a larger group of players per game. Others are much more odd with objectives like carry a gnome all the way to the end of the campaign.
Versus: Personally my favorite gamemode. One team must play as the survivors (the boring team) and try to make it the the safehouse. The other team gets to be special infected and try to kill the survivors (the awesome team). Whoever makes it further as the survivors wins the round and starts the next round as the survivors.
Survival: This is a timed mode in a small map where the survivors must hold out as long as possible before getting wiped out by an incoming horde of zombies.
Scavenge: Surviors face off against a team of special infected to steal gas cans and fill their objective. The team that gets more cans wins.
Realism/Reaism Versus: These modes are campaign and versus only harder and more realistic as the name implies.
Now that you know the objectives, you must know the enemy. The special zombies are playable in versus and each have their own abilities. (Note: Due to my massive amount of research, I can confidently state all zombies have a similar weakness bullets and fire).
Infected: These are the basic zombies that come in many forms. They die pretty quickly and should not pose a threat unless a horde attacks you. A massive horde can attack at prescripted events, car alarms, boomer bile, or just randomly when the game stops liking you.
Boomer: The boomer is a fat zombie that is actually very, very, very weak healthwise despite its looks. It dies with a few bullets but players must be very wary when killing it. Upon death, it explodes releasing bile that will blind the player and attract a horde of zombies. It can also puke on the player which does the same thing. They are very loud and as you become more experienced, you will understand its distingushable sound.
Spitter: Similar to the boomer in attack but about 1000 times as deadly. The spitter only exists for one purpose and one purpose only, to ensure you and your entire team are a dissolved goo on the ground somewhere. She spits a goo at your team that is acidic and increases damage as you stand in it. To make matters worse, she teams up with ally infected to keep you pinned in the goo. Do not worry though, the goo only lasts for a few seconds and she can be seen from a distance due to here lime green aura it causes. Killing her also releases acidic blood so DO NOT melee her.
Jockey: A smaller infected with a hunchback. The jockey jumps on the player (similar to a headcrab) and will ride them into hazardous areas such as spitter goo, fire, or off a ledge which is an instant kill. Whacking him in midair is a 1 hit kill but thats hard to do and not recommended. When a jockey is riding a team member, watch your fire! He's a small target and you will likely hit the ally. Try melee or getting to an angle where you are confident to hit it.
Hunter: Similar to the jockey as it leaps through the air to grab your team. This hooded fiend hides in the shadows and is an expert at blending in since it's black clothing does a great job blending with the game's art design. The hunter will leap in the air and scream. When it lands on a survivor it pins him/her and begins clawing them apart. He does massive damage so eliminate him quickly as he can effectively finish off a low health ally or slaughter a straggler.
Charger: When you see this hulk charging towards you, you only have two options. 1. Shoot him down or two sidestep out of his way. The charger will run at an ally and crush them into the ground until they are dead. At this point, he can only be stopped if he is killed or pushed off the ally. He can be dodged if you move out of his way and he hits the wall. The charger has one massive arm and is one of the biggest creatures in the game.
Smoker: The smoker tends to hide on tall areas or near hazards and will pull survivors away from their friends. He will then begin to constrict them and also attracts zombies to kill the trapped survivor. Survivors can be freed if the zombie is killed or if the beasts tenticle is destroyed. Upon death, he releases a cloud of smoke that impairs the players' vision and speed.
Tank: Out of every zombie in the game, this is the one you should fear the most. The tank essentially looks like the hulk if his skin was pink. The tank will punch players and take out nearly all their health or throw a prop/rock at them and incapacitate them. The tank has more health than the other zombies and usually appears during a horde attack or prescripted events. Fortunately, the tank does not appear randomly so you are safe.
Witch: When you start hearing crying, turn off your light, crouch, and stealth by or you'll start crying too. The witch is always female and looks a lot like the common infected. Yet you must understand the differences if you want you limbs to stay attached. The witch crys and walks around with crimson red claws. She will look angerly at the player with glowing red eyes if scared. If the player shoots, harms, goes near, or does anything else to annoy it, it will become hostile and attack them. The witch will incapacitate the player in one hit on easier difficulties and kill on harder difficulties. You may run into trolls who will shoot it and cause it to go insane. Fortunately, it usually just goes after the agitator so that plan may backfire.
If you want to survive, your going to need to use your items so you don't die.
Guns: What are you doing in an fps/survival horror if you don't know what a gun is! These are essentially metal boxes that release bullets very fast at enemies. When using, you will start noticing the zombies are no longer moving and are laying on the ground, they are dead. Numerous variations of guns exist in L4D2 and it is truly prefrence on which one to use. (Except sniper rifles, they are universally awful). Accuracy can be improved by crouching or adding a laser sight. Explosive and incendiary ammo can be found to increase your team's firepower.
Melee Weapons: Tons of melee weapons exist in the L4D2 universe. Most only vary cosmetically but the chainsaw is the exception. It is more powerful but needs fuel and will run out after some time. Most melee weapons are a 1 hit kill against traditional zombies and are powerful against special infected. However, they are risky to use since you have to get close to the murderous beasts.
Medkit: You will learn to love this very soon. The medkit is a small red first aid kit that is in most safehouses and can be found lying around the map. This item can heal yourself or your team members most of their health bar. It is important to share these and not hog them for yourself since if one team member falls, the others usually follow. These are needed if a team member is dying. (Thier screen is colorless, if not given they will die if incapacitated again or will just drop dead over time).
Defibulators: These can revive a dead team member if he/she falls in battle. These are only needed if the team member is DEAD not if they are incapacitated. These take the spot of a medkit and you can't have both.
Pain Pills: PILLS HERE! These can be used to temporarily heal you or a friend if they are injured. The health will deplete over time even if not attacked.
Adrenaline Needle: Takes the place of pain pills but temporarily increases speed. Adrenaline slightly blurs the screen but in return increases movement and action speed.
Pipe Bomb: An explosive weapon that attracts enemies then goes off after a short time. Takes the place of molitovs or bile bomb.
Bile Bomb: Like the pipe bomb only disgusting. This is a jar of boomer bail and is usually dropped by the zombies in hazmat suits. It attracts a horde of zombies and can be useful for an escape. However, its real power hides in how if it hits a zombie of any type, they are considered hostile to others and the horde targets them.
Molitov: A firebomb that will burn zombies to death. Understand when using that burning zombies are weaker but can spread fire to players. (Note: DO NOT step in the fire or you will light up too)!
Gas Canister: Used in some of the finalies but can also be shot to ignite the area just like a molitov. Can also be thrown with the action key.
Propane Tank: Like the gas canister but explodes when shot.
Just some tips to keep you alive!
Water BAD: You will usually die if you fall into water. (Maybe the survivors were UNSC operatives before the apocalypse).
Cliff Hanger: If you walk off a lege you will be incapacitated until helped up. However, you will not lose health except from being attacked.
Revival: When you are revived from incapacitation, you health is greatly reduced. Use a medkit as soon as you can!
Property Damage: Doors can be destroyed by infected (except safehouse doors) so never assume you are 100% safe.
Keep Quiet: Lack of noise will not disrupt common infected and you can get by them without them noticing.
Spoiler: In The Sacrifice campaign, there is no way for all 4 players to survive when the generator is restarted. (Note: Usually Bill has to go do it so only be Bill if you are willing to fight through a horde).
Characters: Everyone is the same just with cosmetic differences so your character choice has no effect.
Friendly Fire: Allies still take bullet damage and can be killed by it so watch your fire around allies.
We Are In This Together: Stay with your buddies. If you get separated, nobody can help you when a special infected gets their hands on you!
HEADSHOT!: Whenever you get the chance, aim for the enemy's head. It does more damage and in some cases, will even benefit you. The boomer's explosion will be smaller and the smoker might drop its victim if you hit the tentacle part of the head for example. (Note: In realism, this is one of the best and ways to kill infected due to their increased abilites and damage to the body is incredibly reduced. Also, the witch can survive numerous headshots so avoid her).
Saliva: If you start noticing saliva appearing on the screen (looks like waterdroplets), a smoker just tried to grab you but either near missed or is about to pull you! Check high spots or areas where one might be hiding and kill him. If your using a mic inform allies theres one nearby and they might spot it before you.
L4D1: The game ported all the Left For Dead 1 campaigns into Left For Dead 2. If the campaign uses Coach, Nick, Rochelle, and Ellis, its new to the francise for the second game. If the characters are Bill, Francis, Louis, and Zoey, it is a L4D1 port.
Uncommon Infected: All L4D2 campaigns have their own variant of the common infected with its' own ability. For example, in The Parish, SWAT armored zombies will walk around bulletproof except in the back. Another example is the CEDA operatives that were infected in Dead Center that are fireproof. They have the same health as regular infected but there perks make them a bit more dangerous. The survivors will comment on their abilities if you are unsure.
Fallen Survivors: If an ally dies, his gear will drop at his corpse. Unless someone on your team has defibulators, he's not gonna be using it anytime soon. If you need the gear go and get it. He might even be carrying defibulators to save his own life with.
Zombie Survivors: In The Passing, some infected will look like they are carrying various helpful items like throwables, medical kits, and guns. These zombies will drop this upon death. They can't utilize them so don't worry about an infected in a gunfight.
Shooting door: If this seems like a good idea, break a small hole into a door and shoot zombies through it. This sometimes is a great tactical plan but consult with your team first since in some situations it can have serious consequences.
AFK: The game will randomly spawn events such as a horde or a pile of items. I cannot say to never go afk since everyone does it, but don't blame your team if you die due to a random event that you couldn't be protected from.
Throughout the game, some areas can be difficult to new players. Here is a list and how to get through it.
Burning Hotel: Right after the elevator breaks in Dead Center, your team enters a completely burning section of the level. This is the first encounter with a horde and can be very dangerous. Most infected will run through the fire and essentially kill themselves. Follow the path and kill/shove any infected in your way. Under no circumstances cut through the fire. If for some reason you catch on fire, you will go out in a few seconds but the damage will ruin your health.
Car Auction: In The Parish, there is a parking lot full of cars that EVERY SINGLE ONE has an alarm on. Stealth through this and do not shoot or kill a zombie near a car, try to pull him/her away then kill her. Also, don't touch any of the cars. If one of those cars goes off, the horde will attack and in the fray you will likely set off the other alarms making the fight much longer.
Sugar Field: In the return trip in Hard Rain, you make a return through the sugar cane fields. This area is very dark and high winds pull you around. However, thats the easy part. This instance has the most witches in the game in one spot (usually over 5). Try to stay calm and have a keen eyed team member spot them without the light. If you startle one, everyone must concentrate fire on it so it goes down quickly. Be careful though, you might anger the others nearby too.
Sewer alarm: In the Passing, you must run through a sewer system with thousands of zombies infront and behind you. If someone falls too far back, they cannot be saved. (There are unlimited zombies here and you physically cannot backtrack through them all). If an ally gets caught by a special infected, kill it and continue running to the safehouse at the end of the tunnel. Large magazine guns and chainsaws really help in this scenario.
Motel: There is a small motel near a gully in Dark Carnival. You first runthrough can be a bit confusing here. Follow someone through the rooms and make sure you land on each platform going down the gully.
What To Do Now
Now that you know the basics, apply them! I reccomend doing a campaign on solo with bots then going to multiplayer. (Dead Center is the easiest in my opinion). Also, try to help other new players. With this knowledge, you should be able to get respect from the game's veterans if you apply it.
Some random facts about the franchise that won't benefit your skill but are pretty cool to know!
The Screamer: A special infected that was removed. It looked like a common infected but wore a straight jacket and would scream to attract the horde. It's ability was added to the boomer's bile after it was removed.
Cold Stream: This campaign is not officially made by valve but instead by a community member. However, it is on par with valve and was added to the dedicated servers.
CS guns: In some nations, the game was delayed for various reasons. Valve added counter strike weapons ports to make up for the delays.
Jimmy Gibs Jr.: Apparently, very rarely the owner of the car at the end of Dead Center will spawn during the horde event. He is infected and has a much higher health than his regular counterparts.
Dead Rising 2: In Dead Rising 2: Case West DLC, there is writing on the wall in a tunnel that says "Don't Startle The Witch". When you see this, the witch's song starts playing. Also, in one of L4D2's safehouses, one note on the wall is signed by Frank West, who is the protagonist of Dead Rising 1.
Cutscenes: In the original game, there was going to be cutscenes before each campaign but were cut in the end.
Witches: Witches and other infected might be attracted to sweet things due to their large appearance in Hard Rain. This could also be caused by the storm.
Cosmetic Changes: In The Passing, all the infected have some cosmetic changes. Some are pretty obvious (the witch is in a bridal gown) others are more subtle (the boomer is wearing socks).
Boomer/Boomette: The boomer is the only infected (besides common) to appear in both genders. The female was originally name boomette in the files.
Physics!: Sometimes, infected are spawned in to show off the game's engine features. For example, in The Parish a boomer spawns in the resturant and it's explosion is meant to toss the furniture props around.