That explains why it feels like they heal so damn much even when you're super tanky. Did they also heal on evaded attacks? Because I swear it felt like they did against my 75% evasion footmen. Or they just attacked so fast, they still got heals out that often. It's kinda hard to tell what is going on in the mosh pit.
What might make a target invalid? Because it's very common for swordmen to *not* have aggro, none of my tanks have died. Yet all of a sudden a rogue werewolf is just eating the Swordman's face. Same with archers, they swap to swordmen quite frequently.
Also the "floating blob" effect. Where sometimes the "mosh pit" of melee units fighting each other will just start drifting in a random direction slowly. Is this intended, or some weird physics interaction? Sometimes it causes my AoE units to drift into enemy archers (on those 20+ archer waves) and sometimes it causes enemy werebears to casually stroll into my backline and murder it, leading to an instant loss. It overall feels very random.
I think overall attack nearest, possibly with exceptions for units based on mutations/enemy variations is probably a good targeting. There being some way to bypass frontline to hit backline would probably require there be some reasonable counter. Not just taunt, cause taunt kind of either forces you to go knights (and imo knights are just worse footmen due to tempo loss). Or you *need* to roll a rare. Feels like it would lead to a lot of "scheduled losses" if you don't roll into specific things. And if you add it to the enemy, you really should add it for the player as well. Might give archers a niche if they are allowed to attack back to front. But overall I think being able to "cheat" the frontline like that in auto battlers usually just leads to it being mandatory you do so. Or backline units would need to get a lot tankier.