The hardest part about Ultimecia is that she selects three random characters from your party. But I'm sure we all knew how to get around that. Griever and third form Ulti can cause some problems but it's really nothing that can't be Curaga-ed/Recover-ed out of. Also you can't die in the fight at all, because there's no way to revive the character that died. But then again, it's very hard to die in this battle and even if you lose a character, you'll usually pull up fine.

I think I know what her problem is. I've found that final bosses in many RPGs tend to annoy you by bombarding you with status effects. Necron and Safer Sephiroth both did this and both proved to be more challenging than Ulti. Damage could never really be a major concern but that's pretty much how Ultimecia attempts to beat you with moves like Shockwave Pulsar and Hell's Judgement.

Doesn't mean the battle wasn't fun though. It was a nice way to end the game.