Pedals are threaded that way so that they're tightening while you ride. You've got who knows how many miles of tightening you're fighting against to get this pedal off. Make sure you're turning the right way (if wrench is pointing up - between 9 o'clock and 3 o'clock - then it's always towards the back of the bike) and get a big ol' pipe. Comically long. And don't be nice about it. Turn the correct way and it WILL come off.