Berserk Charge + Negative Combo + Explosion on any boss fight.

berserk charge enables you to quickly hammer the boss's HP to next to nothing, but (as i found during the sephiroth fight) It is impossible to kill most bosses with Berserk Charge in Effect (while Mp is recharging) because they must be killed with a finisher. enter negative combo. Two hits = finisher = kills boss.