How do you get to fight sepiroth in Kingdom of Hearts 2?

When you finish the computer place, you meet Mickey. Then you go down there and kill Demyx, followed by a thousand heartless (literally). Somewhere around that place you will be meeting Axel. Go back to the place Axel was later, and you find mister Evil.

He is very very very very hard to beat. When i tried to beat him he killed me in one attack. You will receive a reward of another gauge lvl (up to lvl 7 gauge with no cheats) he is an optional boss soooooo dont stress about it k ok.