Check out the guide on formations in my signature – it has a section about AI behavior on the battlefield.

A while back, almost a year ago, I was working on a similar issue, but only with 1-on-1 units. As an example, I studied how a stack of wyverns behaves when holed up and attacked by dwarves. Given their speed, the wyverns move first and have two behavior options: wait or fly forward completely. This depends on whether they are afraid of getting hit by the dwarves. Ultimately, it involved comparing the values obtained using the formula below, and the ratio between them could be used to understand the AI's tactics.
The formula itself looked something like this (FV - Fight Value from crtraits.txt):
K_stack = HP_unit * (stack_damage / FV_unit), where
FV_unit = sqrt(((FV_base + (hero_attack * FV_base) / 20)^2) * ((FV_base + (hero_defense * FV_base) / 20)^2))
However, this isn't quite everything, but it's an approximate explanation of how the stack simply wants to deal more damage, not just to HP, but to HP * FV, and it also takes into account the hero's attack and defense separately. That is, all other things being equal, a stack with 5/5 attack and defense and a hero with 0/0 is not the same as 3/3 and 2/2, even though the damage is the same. At the same time, the wyverns always flew in if the ratio (K_stack_dwarves / K_stack_wyverns) >= 2. That is, they were already resigning themselves to defeat. Also, if the ratio <= 0.07, they consider that they have already won.
I can describe all this based on my old notes, but it's quite possible that I'm wrong about something; after all, it's been almost a year. If you're interested, I can show you examples of calculations and everything, but I'd have to refresh my memory.
In practice, this translates to: attacking harpies in hives, attacking archers at the beginning of the round against the advanced units, not against elves, the fear of getting hit by a stack of wyverns, and attacking with a single unit.
P.S. And yes, by the way, stack_damage is the average damage. That is, a stack of air units will calculate its damage as 5 with a damage range of 2-8. At one point, I wanted to record a video with the formula displayed, but I was just too lazy.