here is a video guide i hope it helps but my advice use your own strategies to beating him cause he is a hard boss also use a powerful keyblade of one u are comfortable with of like the video says using ultima weapon will make the battle easy.
To beat Sephiroth u need to be AT LEAST level 80 and have ultima weapon. I would 88 of 90 just to be sure u may also want to equip hi potions and mega if u want but i wouldnt advise to since Goofy and Donald cannot be with you. u also want to equip Leaf Bracer. I beat him on level 73 with Ultima but ive also played the whole without dieing.
I suggest having all of the growth abilities...Especially High Jump, Aerial Dodge, and Glide. It's important to be able to reach Sephiroth when he uses Heartless Angel. I saved 6 elixirs for the Sephiroth fight. I only used 2 though. Oh, and don't blink. Never miss a reaction command. Combo, combo, combo. That's the key. Ultima weapon is recommended but NOT required. u can just as easily beat him with Oblivion like I did the first time. Good luck :)
a way to beat him is to use the oblivion keyblade, have a lot of exilers on you, when he is just walking slowly attack him fast, and when he teleporst use gaurd to protect yourself keep doing this and u might be able to beat him!
The keyblade that made the battle the easiest for me was the hades crest from the underworld part of the Olympic colosseum. The reason was that every time u use a combo finisher, sephiroth teleports and usually counters. But what this keyblade does is make to where u don't finish the combos when u are out of mp and u get to keep on damaging him for a lit meer than u would when using combo finishers.