I beat him with ease and I was at level 97 with all my forms maxed. But, it's probably possible to get the job done at around level 80ish.
Try and get the Ultima Weapon and have at least 2 elixers before you go fight with him. There's really no strategy ... well,maybe a little.
All I did was:
At the start press triangle to block his first attack. Then hit him with a few combos to get two or three of his bars gone. Then, stay away from him ! Don't go running after him, or he will counter. Wait till he disappears and appears behind you and hits you in the air. When he does that, quickly press O for your Aerial (sp?) Recovery and counter attack quick.
Next, once he sends the black balls at you, 'quick run' or w/e it's called towards him and attack. That part isnt too bad. Just make sure you Lock On to him when you have the chance.
When he flies up in the air, and says "Descend "Somethig" Angel" (forgot what he says ) Lock onto him, and jump towards him and attack before he has a chance to finsish what he's saying to stop that move. (You'll need High Jump Lvl 3 and Aerial Dodge (I think thats what it's called) lvl 3 to get a good chance of getting to him fast.
Then, when you see he's starting to get a red aura around him, while he's sitting there talking run up and attack him as soon as you can before he can use the black orbs. (He will use them, but if you get to him fast enough and start attacking your "Explosion" ground finish should destroy most of them)
When he calls his meteor attack, there's nothing you can do, but just jump and glide, jump and glide some more. (by this time he should almost be dead) and then, just keep yourself healed, and wait for the oppurtune moment to give him your last combo, and Bam, g'bye Sephiroth.
That's just how I did it.
Pretty much, you need to have most of your growth abilities. So, take some time to level up your drive forms and it shouldn't be too hard.