Ok so you want to know the best way to beat him right. Well I will tell you want you need. First I would say to be at least lv 60, it might even work at lv 58, so you need to level up some more. Second I would have have at least three elixiers. Third have the second Chance and Once More Abilities. Fourth have all you forms leveled up to the max, Fifth make sure Sora's Limit command is on. Sixth for your keyblade, use the Fetal Crest not the Ultima Weapon because you can do endless combos. Once you got all that then you call challenge him. Now when the match starts block his first move, then use your Limit on him, then right after your limit is over just attack him as much as you can before him goes away. Then just keep targeting him while flying around wait for your mp to come back so you can use your limit again. But if you are almost dead use one of your elixier's then use your limit again. Now when he rises up and says Sin Heartless Angel try to get to him before he does his attack, if you cant make it to him then be ready to use another Elixier for as soon as he is done with that, your MP will be gone and you will have only 1 HP left so what you do, is use one of your Elixier's then use your limit again. You should beat him. After you beat him go to Cloud in the shopping area, remember where you started at. After you talk to him go back to Sephiroth to see a awesome video and then you get a new Keyblade. I hope everything I told you helps.