Erm... for KH1, he has an invisible life bar that's 150 points or so. Deplete that and you'll be able to deal some real damage.
I beat Sephiroth at level 99, lmao. I didn't try to beat him at an earlier time. Basically, here's what I suggest you do:
When the battle starts, he instantly jumps into a combo. Press triangle to deflect it, then use Trinity Limit. Don't forget to equip Berserk Charge. Once Trinity Limit is done with Berserk, you have the Reaction Command "Break." Use that to inflict much more damage. Once your magic is gone, you can either glide around the arena and physically pound Sephiroth and wait for your MP to charge, or just use an Elixir. I used Elixirs, because you can keep using Trinity Limit and Break. Keep pressing R1 too so you can quickly find him after he teleports. When he leaps into the air, use Aerial Spiral to close the gap and let Trinity Limit go on him. That will knock him out of the air and stop his most devastating attack, Sin Harvest. If you don't make it in time, you're HP will drop to 1 and MP to 0. Right after Sin Harvest, he'll fly down and perform the combo he did at the start of the battle. Quickly press triangle to save yourself, and then use an Elixir. When he has a wall of fire around him, glide away. The fire will try and pull you in, so again, glide away. When he summons meteors, use Reflega or whatever it is to send them back at him. You can also manually attack them to get them out of your way. Basically all you need to do in this battle is use Trinity Limit. Best of luck, chief.