Nyx Avatar
from the original Persona 3. In Reload, he was nerfed and is a joke now. No more Charm + Diarahan on the last phase unless you let it happen or get really unlucky (can it even happen at all? I'm not sure).
Gullveig from Atelier: Escha & Logy might seriously be the hardest final boss I've ever faced though. I'm sure it's because I didn't bother to learn how to synthesize properly, but Jesus fucking Christ, I can barely scratch the thing and my entire party is at level 50.