How do you get to Sephiroth in Kingdom Hearts 2?

First and foremost, you will have to have sealed the Space Paranoids keyhole in order to find Sephiroth. He appears just beyond Crystal Fissure (the place where you fought 1000 heartless) in the last section of Hollow Bastion, which is now called Radiant Garden, called the Dark Depths. Hit Triangle to converse with him to engage in combat, I recommend your level to be about 85-90ish because he will instantly kill you with one blow.