Some good suggestions here, but my first tool for a stubborn pedal is a machinists' hammer. It may be a scary thought to some, but have found the impact from a modest tap or two on the wrench is usually all it takes to loosen stuck threads. Hold the wrench and crank arm firmly with your other hand while tapping, or get a helper. Use caution of course. If a few reasonable blows doesn't do it, then would try penetrating oil overnight, followed with a repeat with the hammer taps.