Wiseman from Baten Kaitos Origins. Holy shit was that boss hard.
Technically speaking, it's not a 'required' boss you have to face to beat the game, but if you want to get the right ability to face the final boss, the Wiseman fight is the way to go.
Not only do you have to fight a massive Black Dragon before fighting him, with lots of health and devastating attacks, but once you're fighting Wiseman, his every attack saps a significant part of your health and a resource analogous to 'mana,' which determines how many attacks you can use per turn.
And he's got plenty of health, too. And if you can't beat Wiseman in your current state- if you need to level up or somesuch- be prepared to load a previous save and do the fight with the Black Dragon THEN Wiseman all over again.
I barely beat Wiseman in my last save. Just barely.
I'm never fighting him- or her, or it, it's not really clear- again.