How to defeat sephiroth in kingdom hearts 2?

Preperation: First off, make sure you are at least lv 70 or higher. Also make sure you have stocked as many high potions as possible. If I were you, I'd set my potions to shortcut instead of spells. Yes, even cura, because Sephiroth will want to drain your mp a LOT. Next, make sure you have your strongest keyblade equipped. Even if you are an extensive magic user, spells do not work well against Sephiroth, go for strength, not magic.

Abilities: Make sure you have as many combo abilities on as possible. Also equip aerial recovery. If you have abilities like "lucky lucky" or something like that, drop it for fighting abilities temporarily. Also equip finishers and aerial finishers. These attacks will do MASSIVE damage.

Forms: You cannot use forms while fighting, however the abilities from the forms can be used if you have leveled them enough. I would highly recommend having a lv 5 wisdom form and lv 5 final form. You will understand later in the explanation.

While fighting: Don't let it slip your mind that he starts off with his 6 slash attack. Press Δ at the beginning of the fight. As soon as you finish blocking, attack him before he can recover, but back off after your first combo; if you get too reckless then you will be very seriously hurt right off the bat. If he launches you into the air use that aerial recovery you equipped to counter back on him and gain the advantage. Sepheroth's cheapest move is the heartless angel, where he drops your hp to 1 and magic to 0. RESIST THE TEMPTATION TO CURE IMMEDIATELY. As soon as he drops your hp he will go for the 6 slash attack again, block that and then heal! When he uses his gravity fire, use either the glide from your lv 5 final, or your dash from your lv 5 wisdom. when he stops he will attack you again, wait until he finishes to attack again. The little purpleish black balls are really just nuisances, but they can open you up for an attack. Either glide over them, or slash them to get them out of your way. Use the trinity limit whenever he isn't using a special attack, it can prove very useful in the fight.

Reminder: Sephiroth gets harder to fight after you deplete half of his heath. Keep that in mind while fighting. Also make sure you use a potion any time you get lower then 50% health. He can take the rest of very quickly.