To beat Sephiroth you have to have Oblivion or a keyblade that will increase your str. to 80+ or add a lot of power up to Sora. His tactic is mostly hit and run (When he hits you he suddenly runs away and hit you again etc.). When he will say "Sin Harvest" hit him quickly. (To know when he will say that he will teleport very far from you.) When his life points is green he will cast a spell to increase his endurance.(To know it he is a bit blue on his body.) He will also cast a meteorite spell and the meteors will spin and protect him and you can't damage him when he casts that spell. His other destructive move is when he will glide and attack you (of course) as the same as Cloud only longer. If you follow this you have a very high chance of defeating him.