Since the fitting is already buggered you can try vice grips or a small pipe wrench but you run the risk of damaging the trans cooler if your not careful. If you still can't budge it and it's completely rounded off I would cut the line right at the stripped fitting (you might be able to get a socket on whats left of the fitting with the line out of the way) ,pull the radiator, then work on the stripped fitting while the rad is out of the car where it's easier to work on. Then get a new fitting (napa most likely has them) ,re flare the cooler line and done