Main thing that shows is that they managed to have 'hundreds' of bats.
The way I'm kind of thinking combat works when broken out to mechanics is that any stack can engage any other stack that it wants, regardless of if the 'defending' stack is already in a fight with another stack (Less sure about that last part, not sure I can say I recall seeing any cases of 2 stacks v 1 stack, though the fights can look very chaotic). An already engaged stack however can't break off to fight someone else, though they can make an attempt to retreat (possibly using another stack as a screen). Once a stack has defeated an enemy stack or disengaged, they can rearrange their stack with units from other stacks that aren't currently engaged.
So essentially what you get is that you have a few different types of stacks: Assault, Defensive, Support, Reserve. Assault stacks try and pick out targets of opertunity among the enemy, such as unled stacks, casters, or high value warlords or whatever else they might deem is important. Defensive stacks instead focus on stopping Assault stacks from hitting their targets, basically acting as interceptors/walls. Support would be things like archers, healers, casters. They wouldn't directly engage, but would instead use their abilities in whatever manner they could. Finally Reserve stacks would basically be all excess unled stacks which are basically there for the sake of replenishing injured stacks. Technically any Reserve stack could also be a Defensive stack, but would kind of act as a last resort since an unled stack is more or less going to get slaughtered by anything led by even a low level warlord unless they are very powerful units to begin with.
This basically allows me to auto resolve combat in a turn based manner by having the people on each side set up what stacks are what kind, what the Assault stacks should prioritize attacking, what the Defensive stacks should prioritize protecting, what their support stacks should be doing, and the reserve stacks basically just hang back.
Main problem with this is that I feel like you would have a huge snowball in battles, with them basically being determined by who's main warlord lost a fight first. Then again, that would encourage having a large number of warlords (along with them being a means of getting casters), which seems to be the case, given that every other person in the Jetstone army seemed to be a warlord. Being able to disengage might help prevent the snowball, but would diminish the value of multiple warlords quite a bit. I'll have to put some consideration into it.
Oh, and if there is no leadership in the hex at all (barring a few specials and magic) basically all stacks are treated as Assault stacks or Support stacks (in the case of archers)