Honestly I've never had a problem in any of my FFIX playthroughs with Ozma, but I'm also the power-leveling type so by the time I reach Ozma, my party is pimped out as fuck already.

Difficult bosses...that's actually a tough one for me...to choose that is...
Probably Terramorphous (spelling?) from Borderlands 2. So many tentacles, you'll get tenta-graped if you aren't careful.