Ultimecia, like all bosses, is very easy if you resort to cheap tactics, ie Holy Wars and Aura spells (and Invincible Moon).
I try to avoid those whenever possible because seriously, there's not much fun in taking the easy way out.
For me she was the hardest storyline boss though. Honestly, I wouldn't expect anything less. What's the point of having a final boss that's easy, after all. XD Mostly it's the preparation, though. Having the right spells junctioned and the right abilities equipped makes all the difference. I remember getting unlucky, but I still managed to defeat her on the second or third try. I'm replaying at the moment, so I'll be able to be more in-depth once I complete it again.
~DragonHeart~