Wesker in RE5.
I still don't fully understand the mechanics of any of his three fights.
Every time I think I understand what I need to do, something just doesn't work properly.
The first fight with Wesker and Jill is like... on a timer... but you can drastically reduce the timer and get Wesker to leave early if you do things right (and get a better ranking)... but Wesker never cooperates. ...and trying to get the thing off of Jill's chest without killing her is sooo hit or miss.

The 2nd fight involves turning off lights, shooting an RPG with lights off, shooting the rpg after wesker catches it, then having your teammate hold wesker while you inject him. At any step of the way, things just seem to not fucking work randomly... and reloading the rpg is such a massive pain (one shot with it, then climb three ladders to get a rocket, takes 2 players to reload etc.)

Third fight, you need to shoot the little orange dot, but sometimes you can spend all day long shooting it with the most powerful weapon, and he will just take 0 damage, because some sort of scripted nonsense needs to happen first.
ughhh
Wesker is the boss of one of the DLC's as well... and as much as I love the rest of the game, I never even bothered playing the DLC because I never wanted to fight that shitlord ever again.