Ultimecia was incredibly easy. But then, so are most bosses in Final Fantasy games. Chaos was the easiest. I wasn't even done preparing for battle when he fell.

That being said, Necron is the hardest boss I've ever faced in a Final Fantasy game. I haven't played FFII or FFXII and I'm only in X-Death's world in FFV. The other mainstream games I've played through. (And I'm not counting FFIII, for reasons you know if you've followed my posts. I'm not getting into it here because people attack me for reasons unknown when I bring it up.) So, seeing as how I've beat most of the bosses Final Fantasy has to offer, most with no trouble, my authority that Necron is the hardest boss in Final Fantasy is pretty solid.

Jesswee, that happened to me with Atma Weapon last time I played Final Fantasy VI. I beat him just fine. Then I killed myself on that pole you beat on the way out. Rather than checking to see if he had reflect, I cast Fire 3 on him four times with all four of them reflecting back to me and killing my characters one by one with nothing I could do to stop it. The second time I faced Atma Weapon, he was incredibly difficult, but I still managed. I don't know what the difference was.