From Square's Final Fantasy VII, Sephiroth returns in Kingdom Hearts as a optional boss in the Olympus Colliseum. To enter a match with Sephiroth, you have to have finished the Hades Cup as the victor. Then, when you converse with Phil, you can select under all of the other Cups and Matches, the 'Platinum Match.' In all honesty, many consider him to be ultra-hard and ONLY ONE strategy can allow you victory and that you HAVE to be at a certain level. No. Honestly, you can defeat the 'One Winged Angel' at level 40. Don't bother worrying about level, simply worry about your stats and abilities. Preferably, you need to have at least enough time to surivive a one attack.

For preperations, equip a keyblade better than Lionheart or Lionheart itself. The ability, Second Chance. Once you have these things, make sure you have the magic, Aeroga. Then enter the battle. Basicly start and Jump. Cast Aeroga to reduce damage. Heal whenever you can. Make sure you have Elixirs. When Sephiroth casts 'Sin Harvesting' Superglide and beat the crap out of him. Continue this pattern, and damage him with high end abilities if you can. When he uses Meteorain, dodge and then make sure your prepared to block all attacks. If you win, great.