I just stuck with one main party and trained them until I thought they were strong enough. If I found that the other party members were weak in Kefka's Tower and having a fun time getting KO'ed, I'd train them enough just so they'd survive. Or equip Mog with that amulet of his that stopped random encounters. In the end, I only ever take on Kefka with the main party I trained. So meh.