To beat Sephiroth, you need to go past the Hades Cup, and that will unlock a challenge called "???". (Platinum Match after you do it once)You need to have the keyblade "oblivion" or "Ultima Weapon". You need to have Ars Arcanum on, and Second Chance, as well as at least one MP rage. Do not put ragnarok on as it messes you up A LOT. You need to have strength at least 65+. When you start fighting, immediately jump to dodge his first attack. You cant hurt him for a bit, but if you do enough damage his health will start going down. You need to get past the first bar, trying to dodge his sword slashes. When you hit the orange bar, this is where the danger starts. He will yell "COME!" and charge at you.(or use a giant fire spell) Right before he runs into you, he teleports. He will then yell out "SIN HARVEST!" You need press R1 exactly when he teleports to find him quickly, and hit him as soon as possible before he takes all your health away (except 1 small point, and you cant heal). If you fail, run away and quickly use a Megalixer if you have one. This will restore your MP and HP, and allow you to use magic again. If he uses the fire attack, use Ars Arcanum in the middle of the flame. He will repeat attacking you, using sin harvest about 3-4 times, if you kill him within 5-6 minutes. When he gets to the yellow bar, he will yell "POWER!" and blue magic will surround him, and will make an assault of quick attacks. After that, he will call down meteors. When he does this, get as close as you can to Sephiroth, and circle around him slowly to avoid the meteor that surrounds Sephiroth. Then he will call some orbs to hurt you, over and over again. By this time you should have him on the green bar, and then continue hitting him. If you dont do this quick enough, he will use another sin harvest. Eventually you will kill him.

Congratulations, you have killed the hardest boss in Kingdom Hearts 1!