Where do you find sephiroth in kingdom hearts 2?

First go to Crystal Fissure. Then go to the place where you fought the 1000 heartless. Keep going North(Up). You will find a man by the castle(not exactly the castle). Go near him and press Approach(Reaction Command). Sora & Sephiroth will get into an argument. He will challenge you. BUT BEWARE, HE IS HELL HARD. You will need the Ultima Weapon. There is no drives and magic is useless. After you beat him, talk to Leon,Yuffie,Tifa,Aerith or Cloud to get the Ultimate Weapon "Fenrir"