Yunalesca, I must've died about 3 times against her final stage. Fustrating to say the least. When I played it the first time I didn't have 'Holy' so each time i fought her it'd take about 30 minutes to get to her last stage and then be wiped out eventually by either her regening one of my zombies then megadeath.
Eventually I managed it, but not before I said I hated this game.

EDIT: I just noticed you had "Robot In Gorge" A very close contender to Yunalesca, I ended up having to Get all my summons to their Limit breaks and then summoning them, using it and then watch them die. great fun nonetheless ¬_¬