You should really get High Jump, Glide, Quick Run and Ariel Dodge.
These will help you throughout the battle.
Another skill you should equip is Guard Break. When Sephiroth starts slashing, guard, and use Guard Break. This will stop him from slashing, giving you a chance to charge at him.
Here's some tips on how to dodge his attacks:
Ground Slash Combo: As I said, use Guard Break, it will help a lot.
Aerial Slash Combo: Use Aerial Recovery, and follow it up with Retaliating Slash. Yuo will both descend to the ground after you do some damage with it.
Flash: Just use the Reaction Command 'Block'. Thank God there was a reaction command for that, otherwise you'd be Sora Sushi if that move hit you.
Firaga Wall: This is when Sephiroth Summons pillars of fire around him. This is where Glide and Quick Run come in handy. Just use them to escape the pull of the Firaga Wall, since it can suck you in.
Shadow Flare: This is when Sephiroth summons balls of Dark Energy around you. Al you'd have to do is use Reflega, and fire them back. If you're lucky, some will backfire back to Sephiroth
Heartless Angel: This is probably Sephiroth's most devastating attack. It removes all your health (till 1HP left), and all your MP. The worst part is, you can't avoid it. But here' s what to do. Make sure you have an Elixir handy.
Now, right after that he'll use Flash, so make sure you use the 'Block' command. After you've blocked quickly use an Elixir. That's one way what to do, but here's my way on how to do it:
As you see Sephiroth rising to the air, immediately go to the items section and keep it on Elixir.
Don't press 'X'! Use it right after you've been drained. Then, very quickly, use the block command. You'll use an Elixir, and a millisecond after the Reaction Command. OR, you could just attack Sephiroth right when he says it. Make sure he doesn't say angel though, otherwise the move will be successful. Good luck with this move.
Meteor Shower: This is when Sephiroth rises to the air and meteors will hail all around the area. Quite a dangerous move, especially if you're on the ground. Best thin to do is glide around the area, and quick run and glide again once on land.
That's all his moves, and each way to evade them made by yours truly. Those are what I used anyway. Make sure you get loads of Elixirs, they're very important for this battle, since both your MP and HP will be lost.
Good luck.
