Try some penetrating oil. Light household oil will do in a pinch, WD-40 is not the best. Spray or drip it on both sides of the pedal axle to extent possible, leave and go do something else overnight. You can also try heating the crank arm with a heat gun or hair dryer.
Also remember that left side is left-hand threaded. It is easy to get mixed up when using the allen wrench on the pedal since you are applying force to the back side of the axle.