Most people are saying to defeat Sephiroth you need to be level 65-100 but i've done him 7 times at level 50. (Dont ask why i've done it so many times) But anyway my tactic was...
Equip 3 Hi-potions and 3 Elixers to start with and make sure you have Second Chance on.
When you fight him hit triangle (Obviously) and then attack him. Its a good idea to block his attacks except for the one that knocks you into the air. Hit circle for ariel recovery and then hit x to attack him. That stops him attacking you. When he uses the 'give into the dark' thing then hit them away. When he uses the stupid thing that gets you down to one HP attack him beofre he uses it and then use that double jump thing for the meteors. Thats what i did anyway ... Hope it helps =]