Kingdom Hearts 2 Final Mix HD - Sephiroth, Critical Lv. 1 (No Damage, Limits, Drives)

Classic staple fight. Most of Sephiroth's attacks will 1-shot you. If you trigger his revenge value, he'll teleport. If he fakes a teleport (like an afterimage), he'll heavens light you into the air; just block or dodge roll it. The big danger in this fight is if he decides to teleport and do a full combo right next to you, it can eat you alive if you don't react quickly enough.
The first phase is really easy. Just guard and then either reflect or counterguard him into a full combo, and repeat. If you don't hit his revenge value, he'll just keep doing his sword combo. His second phase starts at 4.5 bars, and he starts using magic and running around. Again, just bait him into doing his sword combo as much as you can. You can also attack him if he decides to jump near you. Otherwise, he recovers very quickly from most other attacks so I'd suggest guard+counterguard rather than reflect, it'll probably miss or he'll do another attack. You need aerial dodge and high jump to reach him when he does heartless angel, should go without saying.
Last phase is when he has 4.5 bars left. Always time a finisher to stagger him out of the shadow flare after he powers into the phase. His DM, the black materia with the meteors, you can avoid by carefully jumping laterally. He likes to spam sigma a lot here, even if you're far away. His sword combo now goes across the arena now, so back yourself against a wall so you can actually counter. He'll just end up behind you otherwise and you'll miss.