Have all item slots of elixers. There should be 6. Now turn on Negative Combo, turn off Berserk and any Combo Plus you have. Make sure Combination Boost is on!
First, after the cutscene, hit Triangle repeatedly. Then as he is rushing past you, lock-on and hit X to attack as much as possible. The thing is, get used to his attacks. Jump over him in the air so he swings once, then start hacking over him, you should do your Air Finisher thing. Then he usually ports behind you, so you will be stuck in a mid-air attack, to get out of this, hit Square so you will counter and get some hits in while defending yourself. After you've got his life down about 3 bars, start using the Trinity Attack. You should be able to do it 3 times using Triangle for Break. After it ends immediatly hit him a few times and run away! Tryin' let your Magic refill by itself, but if you get injured or want to attack some more, use an Elixer. The thing is, don't use them all too early. That's why you should have him down at least 3. You don't want to get to the end just to get screwed with no attack. All his attacks you can counter with the Trinity Limit. The wall of fire, dodge away from and try to refill your magic. When he rises into the air, attack him, if you are too late, have your Elixer ready. I was always 2 steps ahead, During an attack, i would make sure my menu was always on what i needed next, Items or Limits. It's just smart to do. The reason you don't have Berserk on is if you are about to win, you can't if you are in Berserk because it doesn't allow you to do a finishing move. If Sephiroth is a dirty whore, and does his Heartless Angel thing, use Trinity Limit right before, your life will go down but you will start your attack.
Be calm, use your head. You WON'T get it the first time, so don't get frustrated. Each time you will get closer and closer, so don't give up aight? You'll win eventually. Good luck. I onyl beat him this week so meh.
I was at level 70 btw. Use Oblivion and be at least level 5 on all forms. You need the double jump, dash and glide reeeeally bad here. And use Strength Boosts.