What level do you have to be to defeat sephiroth on Kingdom of Hearts 2

This really depends on the player. I've read walkthroughs stating that you had to be at least lvl 80, while I beat sephiroth myself on a much lower level. (Something around lvl fifty, if i remember it correctly). Some people are even able to beat him the moment he becomes available, after only 2 or 3 tries.
So the answer is that there isn't actually a given answer to this question. The higher your level, the easier it becomes. If you really have a lot of trouble dealing with him, you should read some walkthroughs or watch some videos about the fight. They can be a great help too.