Where is sephiroth in kindom hearts 2?

You can only find Sephiroth in Hollow Bastion, first time in Postern, the last time is when you fight him, hes at the place where you first see Xemnas, when you beat the 1000 heartless, you can only go there after that, You fight him, Cloud comes and help, you get Fenrir (the strongest weapon in the game) and you get a max drive gauge up to 7.
Hi
If u want to fight him go to a big space at the very end of Hollow Bastion, and Sephiroth will be there after u finish Space Paranoids and Hollow Bastion has been renames Raident Garden. When u beat him u will get the Fenier Keyblade. If u want to beat him u should be level 70 or above. However if u have put all the drives up to max you could do it at level 50. I would suggest level 70 anyway just to be on the safe side. Anyways I hope that I could help u.
Cya