It's easier to beat Chac oversouled because otherwise he doesn't give you a chance. Not oversouled the fight goes like this: Petrify (which always hits), hit, heaven's cataract, and you're dead. At least, that's what it went like for me, about 20 times, until I got tired of it and went to oversoul him.

Edited to give thanks to gamefaqs, without which I'd still be trying to beat Chac