The troop stats that you have are the following:
Life: How hard your troops are to kill.
Defense: A modifier that makes your troops slightly harder to kill. To a first approximation you can ignore it.
Attack: How much damage your troops do when they are in contact with the enemy. This is modified by Twilight Warfare and all attack bonuses (gargoyle, etc).
Ranged Attack: How much damage your troops do when enemies are in range, but have not reached you. This is modified by Twilight Warfare and all attack bonuses (gargoyle, etc).
Range: How far away units can be and still be hit by your attack. This is modified by Spectral Vision.
Speed: How fast your army marches to the battlefield, and on the battlefield. This is boosted by both research in Endurance and Selective Breeding. (Yes, I know that Selective Breeding does not say it applies to all troop types. It actually does.)
Load: How much your victorious troops can carry home. (Not part of combat.)
Upkeep: How much blood your troops drain per hour. (Not part of combat.)
With this in mind, combat works as follows. The size of the field of battle is set by the attacker, and is the longest range plus a random number that is probably about 500. The attacker starts at one end, the defender at the other.
Combat proceeds in rounds. In each round the attacker goes first, then the defender.
When a player goes, every troop (almost, see the note at the bottom) that is not able to attack the enemy moves forward as far as it can. Then they all attack. They do random damage that is less than their full possible attack. Then your turn ends.
This is common with all Kabam games. What changes is targeting. That is, if attackers can attack 2 different troop types, what do they choose to target? This is somewhat random, but so far it seems to be that you target vamps, then ranged troops of various kinds, then grunts, then other troop types.
The net effect is that there are two types of attacks to understand.
Speed: This is when you have bats and melee troops only. Then the bats cross the battlefield on the first turn and get to attack. In this kind of attack your ranged troops are useless and die very early. It takes surprisingly few bats to wipe a lot of ranged troops, beware!
Ranged combos: This is when you combine ranged troops (like snipers) with other stuff. What happens is that the different troop types spread out, and defenders have to go through various layers of attackers before they reach the ranged troops. Conversely the defender gets to spread out. Since every troop type has a different speed, you wind up with many layers of attack and defense. Thus every troop type can serve as a "speed bump". And some of those bumps may have a lot of troops, serving as a "defended speed bump" aka a "meat shield". Ranged combo attacks are the most devastating possible attacks in the game.Note that in a ranged combo the bats are always in front, and therefore first to die. And if they are attacking they do not manage to reach the enemy during the first turn, so do little for you. Hence the common advice to not send bats with snipers.
Note: there is special logic for flickerforms, trucks and armored trucks. Flickerforms sit in the back and do not participate in combat until something kills them. Trucks and armored trucks will move until they are in front of ranged troops, but then do not continue moving forwards. Thus flickerforms are useless, while trucks and armored trucks can serve as a meat shield for ranged troops.
That is, if attackers can attack 2 different troop types, what do they choose to target? This is somewhat random, but so far it seems to be that you target vamps, then ranged troops of various kinds, then grunts, then other troop types