You need Second Chance and Last Chance to ensure some protection.
Leaf Bracer to ensure that your Cure becomes unblockable.
Make your Combo as LONG as posible by adding Combo Plus', and attacks.
Trinity Limit is a MUST have (It is the BEST way to deal Damage.) and a limit plus.
Your Strongest Keyblade Needed (Ultima Weapon is choice.)
Glide, Quick Run, High Jump and Double Jump. (Leveling up the forms.)
Retaliating Slash is needed when you get knocked into the air. (Sephiroth will NOT be able to fight back in the air.)
Beware when the screen gets dark, you must NOT jump you must keep pressing triangle. that is his 100-Slash wich will tear your HP to shreds!
Whenever he floats into the Air that is Heartless Angel Hit him just ONCE to stop the attack. (If you don't your Hp will become 1 and your MP 0.)
Elixers work WONDERS here! just be sure Sephiroth isn't coming after you when you decide to use one.
Whenever Sephiroth Summons Firaga Wall (A Bunch of Fire Pillars) Run as FAR AWAY from it as possible (Quick Run and Glide will hlep you!)
Sephiroths Meteor attack is Devastating! when he prepares to attack press yourself against the wall on the Opposite Side of Sephiroth and when he beings shooting Double Jump to the Left and Glide to the left then Double Jump to the right and glide, repeat untill he stops.
When Sephiroth has about or less than 1 Hp Bar USE TRINITY LIMIT. this WILL kill him. (Being it normally takes about 3/4's of 1 Hp Bar.)
Doing so will earn you the Final Keyblade "Fenrir".
Good Luck he's tough. (I beat him at level 35 with Sweet Memories. NOT a smart move!)