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.