Actually, the only one that really hurts is Azi Dhaka or whatever. You can miss him ENTIRELY if you don't go through the last bit of the game the proper way (but where you can still turn back)

Odds are if you're missing an oversoul monster, that's him.

Force your way through and skip the piano playing, and you should have to fight him. I always forget to do this. I hear he's hard though, so good luck with that.
(but if you're fighting in the via infinito that should be a moot point)