this isnt all you need.
he kicked my butt at least 16 times before i figured this out. when u customize sora with items, it does not take them from the item stock, it takes them from soras items this means when u equip items to sora, set them all to elixers, then go to customize and set any button to elixer, make sure auto reload is on for both items and customize. you should have at least 3 other open shortcuts, set one to cure, and one to reflect, use the other one for a hi-potion or ether.
next u need to be at least lvl 80 and have ultima weapon. keep thiese in mind:
1)when everything goes dark and the "block" reaction command appears you must be on the ground to use block. if u cant get to the ground, immediately use reflect
2) when seph rises into the air and says "sin harvest ANGEL!!", you MUST hit him, this attack will eliminate all HP and all MP. if u cant get to him, and see a cloud of dark around sora, be ready with an elixer, for he will start attacking again just after sin harvest
3)when he conjures up a bunch of dark balls, use reflect to get them off ur back.
4) i beat him with reflect, he was on his last HP bar, i was in mid air and he made everything go dark and i used reflect and the counter attack wiped out the entire bar. shows you how useful it is
hope this helps, post again if u need more help.