I was at lvl 90 or so on Proud mode when I beat him the second time. The first time I beat him was on easy, but I cheated by using Max Health and Max Defense cheats. I still had a hard time trying to beat with the cheats on.

But I thought the only way I could redeem myself was to beat Sephiroth with the Kingdom Key on Proud Mode. So one time, I beat Sephiroth on Proud Mode, no items, equipped with the Kingdom Key.
Now, what I recommend
Be at least lvl 80. If you're playing on normal or Proud mode, make sure you can use Glide lvl 3 and Airial Dodge lvl 3 as normal Sora. I did NOT use Berserk Charge, I equipped MP Rage and 2 MP Hastes while having Ultima Weapon, no combo add-ons (both air and ground), and I equipped combo negative (decreases maximum combos by 1). I used glide during the meteors part. It is true you can stop his Halo attack by hitting him before he casts the spell. I also glided away from his fiery pillars of doom.
lol Have Curaga as a short cut and have six elixirs equipped to you. Also, with my strategy, you need Airial Recovery. If needed, synthesize a bunch of attack boosts and defense boosts. One time, I took down one of his bars in 6 hits.
When he knocks you in the air, airial recover, and when he appears, lock-on quickly and attack to try to intercept him. When he's about to do his deadly sword slash (with reaction command), be sure to gain some distance between you and him because after the attack, you won't have to run after him to attack.
Hope that my and Karamuucho's strategies have been helpful for you. Good luck!