Considering the fact you have 8 members, 4-2-2 party combination is the best. There are three spots that you can block so that way you don't need to move anyone to defend Banon, and one spot gets A LOT of traffic (there is where your party of 4 should go). Celes/Gau, Terra, Edgar, and Sabin are the best without doubt. Anyways wait until everyone is gone and then defeat Kefka.