I've used leg muscle to loosen hard-to-turn bolts by laying on my back and pushing the breaker bar with steady pressure using my foot. Legs > arms. Of course I don't go 'round doing that on 1/4" bolts. As jschmid mentioned, tapping with a hammer helps too. Tap gently and from different angles. You might try a little penetrating oil too.

BTW, when you put in the ATF, make sure to NOT overfill.