All of his moves are able to be avoided and countered...
Alway look out for the Reaction Command, and when you block it, he's open for a Combo
When he's doing his normal 7 hit ground combo, all you have to do is jump up and hover and he'll miss you, giving you opt time to throw in a Combo
When he knocks you into the air, time it right with Retaliating Slash and you can bust in a few combo's before he teleports away
When he does Sin Harvest/Heartless Angel, all you have to do is double jump and glide over to him and attack him before he can cast it, I personally find this to be the easiest move to interupt
If you get hit with the spell though, don't bother with the potion right away, because immediatly afterwords you'll have to use the Block reaction command. After succesfully blocking it, then you can use an elixer to heal and get your MP back
His meteor attack will land a couple hits on you, but overall you can survive through it easy. Merely double jump (Aerial Dodge) left and right the entire time, even if you get hit with a meteor, the Aerial Dodge prevents you form taking damage whilst you are spinning, so you shouldn't really take damage at all
When he summons those black orbs, I suggest you temporarily ignore Sephiroth and concentrate on hitting them away so that they don't get in the way. Whilst doing this, I suggest looking out for the Reaction command, since you should have to use it at some point when knocking away the black orbs
When he glows red and continually summons black orbs, just glide over to him and beat the crap out of him, this way he won't be able to summon them
