This gareenteed works..i just did it...(you do only have 7 by the way)

Party 1: Celes, Terra, Cyan, Sabin
Party 2: Gau
Party 3: Edgar, Locke

Just send out party three by itself, leaving Party 1 at a good crosspoint...
Spam Noiseblaster with Edgar and Steal/Attack with Locke
The guys will kill each other and eventually you can take them all out with just this...
Then send in Party 1 to kill Keffka, using Celes Runic, Terra attack, Sabin Aura Cannon/Bolt, and Cyan Attacking with Genji Glove...
It works really well...

P.S: If you are wondering what i do with Gau, it starts with an S and ends with an e and has a uicid in the middle