The non-drive side pedal is left-hand thread. If you can remove the arm from the bike, with the pedal attached, and clamp it in a bench vise you will be able to get better leverage. Use a hex socket mounted on a breaker bar, and cheater pipe if needed. With the arm clamped in the vise, pedal pointing down, hex socket inserted from the back side of the arm, turn counter-clockwise to loosen.