I don't really think Levels matter, as long as you're over level 50, because I beat him at around 55. Just fight him a lot and you'll learn how he moves.

1st Phase: He has a certain pattern of attacks. Learn this pattern, and this part will be easy. You can't block his attacks, so don't bother. What I like to do is constantly dodge roll. Once you get in a full combo, Super Glide away, because he'll use Flare.

2nd Phase: Not all that much harder. He'll start using Omnislash. You'll know when he's about to use it because he raises the Masamune up into the air right before he uses the attack. At this point, waste no time and immediately use Aero/Aerora/Aeroga. There's no point in trying to hit him during Omnislash because he is invincible. Just keep dodging his attacks, which shouldn't be too hard. When he stops and jumps into the air, go far away. He'll also start using Sin of Heartless Angel, if that's what it's really called), which drops your HP to 1 and MP to 0. Before he does this, he teleports to one of the edges of the arena and you'll hear him say the name of the attack. At this point, if you're about 5-7 steps away, use Strike Raid. If you time it correctly, you should be able to get about 4 clean shots at him with Strike Raid. Don't worry, because you're invincible while using Strike Raid. If he's too far away, use an Elixir immiedately after your HP and MP drop because Sephy'll quickly run at you and slice you up.

Almost there!

3rd Phase: He'll very, very rarely, or probably even stop, using Sin of Heartless Angel now. Sephy'll use completely new attacks now. You'll know that he's in his 3rd phase when he casts Meteor. During this spell, a number of meteors will fall into the arena. The meteors will then move around the arena and you'll have to dodge them. Sephiroth is in the center, but don't try to hit him, because he's invincible. Just use Aero/Aerora/Aeroga and do your best to dodge the meteors. He will then cast Ultima, which basically hits 80% of the arena. If you're on Levels 50-70, I suggest you use Curaga immediately after the attack. He will then start teleporting all over the place to hit you. Keep casting your Aero spell and attacking, and he should end his 3rd Phase.

4th Phase: He will now become stronger. He'll have a blue aura about him and he does the same things as his 3rd Phase. Only problem is that he'll start using Sin of Heartless Angel. Just keep Aero on, keep attacking, and you should win in no time.

The best Keyblade to use during this battle is the Oblivion, if you don't have the Ultima Weapon. After the first battle, it's impossible to lose, unless you just joke around. After 3-4 battles, I was able to beat him with the original Kingdom Key. Good luck, and I hope that helped.