I keep hearing sepiroths most annoying move is when he knocks you into the Sky. I had no problem with that. ANd for anyone having trouble with Sephiroth heres some things I did to beat him.
First Fill the Item slots with Elixers and Set Elixer to one of your customization slots.
Get Ultima weapon.
Have as many Combo pluss's you can equip.
When he knocks you into the air, recover when hes close and hit square to arial sweep him, followed up with an air combo and a second one if he's still close after the first.
When he does that attack that kills off your HP and MP, gaurd agains his stabby attack and if hes close swiftly attack with a combo or two until he goes away and then quickly use an elixer, but if he's not close immidiatly use Elixor since he will most likely attack soon.
When he does the Meteor attack just run back and forth jumping and you should dodge it all.
I have no effective move when he flys around yoru head attacking rapidly with the sword.
Hope this helps someone.