I not only killed him, but i stole all his stuff too!
Once i stole the last thing Zidane switched over to staight Thievery, Quina alternated between Magic Hammer and Frog Drop, Eiko kept casting Madeen or giving ethers to the people Ozma did Absorb MP on, and Amarant cast Aura every time anyone got killed and did Chakra to restore MP the rest of the time. Ozma was spending over half his actions casting Curaga and Abosrb MP and didn't really have much time to do anything to us, so it was just a long process of wearing him down with intermitent bouts of panic when he occasionally took a break from healing to cast Curse, or in one very scary case a Meteor which luckily hit right after i'd finished casting Aura on everyone.
In reward for this, i got a pumice, a "strategy guide" and a really crappy card with only three arrows =P However i am _NOT_ going through that again just so i can get a decent card.
Unfortunatly it seems like the new treasure isn't enough to increase my treasure hunter rank from A to S, and i don't think there's anything else i can do to boost it at this point =/