I've faced seemingly impossible odds in GA a few times and won nonetheless. Sometimes through no doing of my own when my opponent never attacked me. But I've also won a few times by putting my best squads on defense. My opponent did the same and had nothing left to take out my defense teams, whereas I had enough half-geared fluff to zerg down one of his teams in 5 or 6 battles. So just because your opponent has 3 times as many zetaed g12 characters doesn't mean you don't stand a chance. Always make an effort and the outcome might surprise you.