How do you make cloud fight sephiroth in kingdom hearts 2?

May seem impossible but, you have to defeat sephiroth in order for them to fight afterwards. i won against him at LVL. 79 reasonable easily, but keep in mind, i had the Ultima Weapon so, it made my MP charge a lot faster XD.
After you've defeated him you get:
Fenrir Keyblade
+1 Drive Gauge