This should help...
Guide on How to Beat Sephiroth-
Part I-
Sephiroth in Kingdom Hearts 2 is basically just like before, though he is much easier. Plus with the combo boosts, Sephiroth in Kingdom Hearts 2 is a breeze, you just need the right strategy. alright, first, as always, test yourself. Fight Sephiroth just to see if you stand a chance against him. I'd recomend anybody under level 75 not fight Sephiroth yet. Train until you reach a good level and make sure all combos are equipted. Have High Jump on. You equipt High Jump by leveling up with Valor Form. Also equipt Glide Lv2 by leveling up with Final Form. Equipt either Ether or Elixer to beat Sephiroth's moves.
Part II-
So you've trained and you actually stand a chance? Well, now it's time to prove it. Be ready, as soon as you begin the battle hit the triangle button and Sephiroth's back will be wide open. Hack away at Sephiroth's back and make sure you get a complete combo in. Sephiroth might use this reaction command more than once, so be ready. Once you run, Sephiroth will most likely appear next to you. Don't worry, this is an advantage on your part. Let Sephiroth throw you in the air and wait for the right time to hit the square button and then put in a complete combo. After you do that, he'll either try that again, just hack away or use the reaction command from earlier. Follow this guide until you make it to the second part.
Part III-
Once you use the first part of the guide, then Sephiroth will go to stage two. Now balls will appear all around you. When this happens, make sure your target lock is off. Then hack away and you should be able to knock all of the balls away. Watch out for Sephiroth's reaction command and his fire pillar move, which sucks you in. Whenever he does the fire pillar move simply glide away from the fire and wait for him to calm down. After that, follow the same strategy from before. Now Sephiroth will do something brand new. This is the stage where Sephiroth will perform his Soul Harvest move. This attack will bring your MP gauge to 0 and your health will go down to 1. If you are close to him, and he says "Soul Harvest Angel" then get ready to use Ether or Elixer. If you value Sora's life, then use your Elixers or your Ether as fast as humanly possible. If done in time, Sephiroth will use his reaction command. Quickly press the reaction button and then glare at your MP bar. If your MP bar is not fully restored, attack Sephiroth and use the square button to lengthen your combo.
Part IIII-
If you have done the above properly, then Sephiroth will suddenly get mad. He will be glowing red, and he will glide around the field. Do not underestimate him. Now you should be at your best. Once Sephiroth stands on the outside of the platform, heal, if possible and prepare for the next move. Meteors will form and be thrown at you. Don't bother attacking Sephiroth because you can't at the moment. Wait for the move to stop and then stand away from Sephiroth. Sephiroth will now use all of his moves. Hack away at Sephiroth, this is not the time to take your time. Even if Sephiroth uses the pillars of fire, attack crazily.
Spoiler:
You have beaten him. He will now request to meet Cloud. Go get Cloud and come back to Sephiroth. They will then fight and disappear. Tifa will then give you Fenrir, a strong sword that decreases your combo by one.