Well the most I did was beat him with a buster sword omnislash but no KotR.But he had already cast his wall spell and one slash only does 2000 damage or so, so it's better off killing sephy with limits(I set Barret and Cid's to level 3 Ungarmax and Big Brawl respectively) and if possible just Omnislash him with his wall spell still on,it won't hurt him that much and you can see more summons of Supernova.