How do you fight sephiroth in KH 2?

First, since Sephiroth is a optional boss, you need to get as far as you can in the story without the last bosses. Then go to the place where you fought the 1000 Heartless, then you go to the place where you first meed Xemnas and Malificent comes and helps you. There you should see Sephiroth near the end of the cliff, if you talk to him you'll get into a fight with him.