I agree with the penetrating oil suggestion. Also, try some heat, from a hair drier or heat gun. It can help the oil penetrate more easily, and the aluminum arm will expand faster than the steel pedal spindle, which can also help loosen things. Finally, if you have a securely mounted bench vise, you can remove the arm from the bike, clamp it in the vise, and use a cheater pipe on your wrench to get more leverage, or an impact socket from the back side of the arm. This is easiest with the non-drive side arm, but can be done on the drive side as well. Use soft vise jaws to avoid marring the crank arm.