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△ for a short cut-scene. He will use this at the beginning of battle, and further into the fight. When you are finished with the first strike press △ quickly to attack him again. After, you can still attack him with the ✕ button. Use either Upper Slash or Finishing Leap to send him in the air, then keep on with your attack, adding a couple Horizontal Slashes along the way. He will then do either one or two things.  He will stand his ground and attack using his sword. If he does this, jump and use Arial Dodge, then Glide away from him. When you get on the other side, land because you might need to press △ to guard again. He will teleport and knock you up in the air. If he does this, press □ to use retaliating slash when he reappears, then continue with your combo by pressing ✕ until either he disappears or you finish. At anytime after knocking out about 5 bars, Sepiroth will use Heartless Angel. This attack is one that will summon a halo around your head to bring you down to one HP and no MP (With second chance or Once More) or Auto-kill you. To stop this, attack him before he finishes saying "Angel".  If you do not have the necessary abilities to reach him, and therefore cannot attack him, a well timed cure spell will cancel out the attack, but you will still have zero magic. On top of this, if you do not have the second chance ability, curing will make no difference as you will die before your cure spell has any effect. If you don't stop him in time, he will teleport and use either an attack you will need to block with △ or his regular attack. After this, Sepiroth will continue using Heartless Angel periodically. After Heartless Angel, he will become more of a threat, running instead of just standing there and gawking or walking around. He will not only use his standard attack, but he will use something called Shadow Flare, which is a big fire pillar that sucks you in. Just use Glide for it or if you have the MP, use Trinity.  Often there will be dark balls that appear around you. You can either destroy them (it takes only one hit), or use Reflect on them to knock them back toward Sephiroth. The number of balls depends. Sometimes there will be only 3 other times 7 or 8. Also sometimes he will use one of his Flash attacks where you need to press △. If this happens and the balls are around you press △ and the balls will be blocked too. Also worth mentioning is that aerial dodge can knock the balls away. When you get him down to about 4-5 bars of life, He will teleport and turn into his final form, where he is at his strongest and will use a meteor spell to try to kill you. When this happens, double-jump and use glide. Don't stop moving.

Summary:
Approach Sephiroth and press . Use the reaction command △ to block Sephiroth's attack. Select immediately Limit then execute Trinity. Attack. Wait for both you and Sephiroth to drop to the ground. Be attentive. Follow Sephiroth. Prepare for the final.