This post explains the textbook tactics I use, derived from historical models.

Empire Traditional[i.imgur.com]
-->Move A[i.imgur.com]
-->Move B[i.imgur.com]
This is a tactic used by infantry-focused armies during the late middle-ages. Spearmen form a solid first line, with Swordsmen behind them. As soon as the Spearmen break or get tired, the Swordsmen enter the fight and let the Spearmen reform and rest behind them. Repeat throughout the battle. Crossbowmen at the flanks fire at the enemy before the armies clash and then A) fall back if threatened and let the Swordsmen intercept the enemy coming at the flanks or B) move around the flanks and shoot into the enemy's sides and back. This move should also be carried out as soon as any flanking enemies have been dealt with. Alternatively, the Crossbowmen may be used to deal with enemy ranged troops. Keep in mind that Crossbows are at a disadvantage to Bows at long range, so it may be necessary to close in with enemy Archers.
The (optional) Halberdiers are brought into the fight when facing heavily armoured enemies or if breakthroughs occur. The (optional) Knights may intercept enemy cavalry, move around the flanks and threaten enemy ranged troops or stay in the background and perform a counter-charge in case the battleline breaks. A voluntarily created gap in the battleline may also be exploited to perform a charge into the core of the enemy formation and quickly turn the tide.

Empire Modern[i.imgur.com]
-->Move A[i.imgur.com]
-->Move B[i.imgur.com]
This is a tactic used by armies during the early modern age (16th & early 17th century). Pikemen and Handgunners form the core of the army. The Pikemen are arranged in a checkerboard formation to allow for maneuverability and room for supporting troops. Handgunners are at the front and will weaken the enemy before the lines close. The goal is to get two salvoes out before Move A is initiated and the Handgunners retreat behind the first line of Pikemen. From there, they will be able to support the Pikemen by firing into the melee through gaps in the formation. If enemy troops flow through the gaps and threaten the Handgunners, the second row of Pikemen moves forward and closes the gaps (Move B), which will prevent the Handgunners from firing unfortunately. Halberdiers from the reserve are used to deal with armoured enemies. (Optional) Greatswords serve as heavy damage dealers in the heart of combat. Artillery is used to force the enemy into advancing towards you and to weaken and disrupt the enemy formation during the advance. Light cavalry harrasses the enemy during their advance, attacks vulnerable targets such as ranged units and artillery, intercepts enemy light cavalry and pursues fleeing enemies. (Optional) Crossbowmen at the flanks balance the Handgunners' lack of range and, as seen in Move A, protect the flanks from light cavalry within their fields of fire.
Due to the Pikemens' high resistance to charges, their high effectiveness against cavalry, the Handgunners' devastating effect on armoured targets and the artillery's damage potential at long range, this army composition and tactic is extremely effective against armies relying on heavy cavalry.

Bretonnia[i.imgur.com]
This is a tactic used by pretty much all armies for the greatest part of the middle-ages. Footmen with varying arms to counter a wide range of enemies are arranged in a protective ring around a core of Archers, effectively forming a mobile fortress on the battlefield. Heavy shock cavalry (Knights) charge into the enemy line and deal heavy damage, either routing the foe with this initial move or breaking off the attack and retreating into the "fortress" to regroup and rearm (new lances, fresh horses, etc). The enemy approaching the "fortress" is weakened by arrow fire and held at bay by the Footmen. In their weakened state, they are either ground down in the melee, or the Knights perform a second charge. This process may be repeated several times.
This tactic may also be used by Imperial armies. Lorewise, maybe Stirland and especially Averland will probably do so, being rather traditional and slow to adapt new ways of life.

Dwarves Traditional[i.imgur.com]
-->Move[i.imgur.com]
This is based on the tactic used by armies in the early middle-ages, before shock cavalry became wide-spread. The army is arranged in a solid shield wall, with the best troops in the centre and younger warriors at the edges. Behind this protective barrier are heavy hitting but vulnerable troops waiting to engage in melee during a later stage of battle. Shielded Quarrellers are positioned in the front, from where they will be able to shoot about three salvoes into the enemy before retreating behind the shield wall (Move). From there, they will fight enemy ranged troops and wait for the enemy battle line to break, at which point they will move to the front again and shoot into the retreating enemy, either keeping them from rallying or simply decimating them. (Optional) Grudge-Throwers force the enemy into the offensive and disrupt the enemy formation.

Dwarves Modern[i.imgur.com]
-->Move A[i.imgur.com]
-->Move B[i.imgur.com]
This is a mix between the traditional Dwarven Shield Wall and the formation used by modern Imperial armies. The Elders are not going to like this, but it is effective. A line of Thunderers is set up in front of a line of Longbeards (or simple Warriors) with gaps in between. Behind these, another line of Warriors is waiting in reserve. After firing two salvoes into the approaching enemy, the Thunderers retreat behind the first line of Warriors as soon as the enemy closes in (Move A). During the ensuing melee, the Thunderers fire into the fight through the gaps in the Shield Wall. Should the enemy attack through these gaps, the second line of Warriors moves in and closes the Shield Wall (Move B), which will however prevent the Thunderers from firing. Artillery in the back forces the enemy to approach the Throng, rather than them using their Bows or Crossbows, which have greater range than the Handguns used by this army, to achieve the opposite.

Undead[i.imgur.com]
This tactic is quite simplistic. The core of the army is formed by Skeleton Warriors with varying arms, simply arranged in a deep battle line. Archers at the back support the formation and Graveguards form an elite centre. Zombies in the front serve as a shield against cavalry charges or ranged attacks. In melee, they serve as a distraction and to tire the enemy. Ghouls, Dire Wolves or Fel Bats roam the flanks of the army and attack targets of opportunity. The goal here is not a quick victory but a drawn out battle in which the (mortal) enemy will slowly exhaust themselves and be worn down by the tireless horde.

Greenskins[i.imgur.com]
-->Move[i.imgur.com]
Quite unsurprisingly, this tactic is about as simple as it gets. Orc Boyz of various armament form a large mass of bodies, with the biggest and 'ardest ones in the centre and front of it all. At the flanks, Goblins form an expendable shield. Archers are left at the back of the formation. The Horde runs towards the enemy and engages in melee as quickly as possible. The Archers stay behind a little and shoot at whatever they can. Goblins move around the flanks of the enemy army and attack all sneaky like (Move). This may also be done with the Goblin Archers. (Optional) Boar Boyz rush the enemy and throw their line into disarray before the main body crashes into them.
This is extremely brutal and results in staggering losses for both sides. It's also rather fun to look at. Waaagh!