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hmm..you didnt say what mode you are in so...okay
.level up(around 65-99)
max out your forms(you need glide to get away,high jump and aerial dodge to get up to seph,uhh...)
bring elixers and ethers or hi-potion(put elixers and hi-po/ether in the quick commands along with curaga(cure) and reflega but its up to you)).
be alert(yes you have to be coz of the sin heatless angel attack).
you definately need second chance, leaf bracer, once more and berserk(if you have it)
equip your strongest keyblade or ultima(if you can get it b4 facing seph)
He starts with a reaction command,so be ready to press triangle.if he upper cuts you and he teleports next to you do retailing slash and contunue to hit him
when he says "sin..." quickly go to him before he finishes his incantations and hit him. but if he hits you with that he would do the thing when you have to press the reaction comand or you just guard and heal quckly.use reflega if needed or if necesarry.
maybe you should try and find a guide or go to youtube.