Randomly beat Sephiroth this morning without really trying - and I didn't have Ultima Weapon, either... though you don't really need Ultima Weapon anymore when you've reached level 95 (I leveled up a LOT unlocking and beating all the Coliseum time trials). Cloud vs. Sephiroth scene == BEST SCENE EVER.
Now, just have to kill Unknown (even at level 70, he's not particularly hard, but that damned roulette attack... -_-)