Ready for a long explaination on why FE5s AI is dumb and does that? Here we go…
First things first: The game determines who to target based on their current HP stat and the one with the most HP gets targeted as long as they meet 3 other requitements (explained a bit below). Leif is a special case where his current HP is treated as 1 since it just skips part of the routine and saves his deployment slot (which is always 1) over it. In addition to the HP the game also checks if the unit has a higher magic stat, if the target already has a status (in which case the next target gets chosen instead) and if the target has the Immortality skill. If all checks are met, the enemy casts their status staff.
The main issue llies within the allegiance check though. It loads in the first unit of each allegiance and checks if the current selected unit can target that allegiance. Enemy using berserk on a player unit? All fine. Enemy using berserk on another enemy? Don’t do that… unless the first unit of the allegiance is berserked…
The first unit that gets loaded in the final chapter turns out to be the sorcerer in the bottom middle. So berserking that one makes enemies target players… and enemies as well since it only checks the first deployment slot and not the unit using Berkserk.
Another fun example:
Berserking this sorcerer in Ch24 while keeping him away from other sorcerers (so he doesn’t get poisoned by their Jörmungandrs, overwriting his berserked status)…
…leads to the Silence staff sorcerer to silence one of his allies…
…and also the Sleep staff sorcerer to sleep Galzus, making him unable to be recruited.
TLDR: FE5 dumb.