Remove the crankarm with pedal stuck in place, and put the crankarm in a vise.

Take the pedal apart, if it is one that can be disassembled, so that you have just the shaft left stuck in the crankarm. If its not one you can take apart use a hacksaw cut the pedal off, leaving as much of the shaft as possible.

With the crankarm in a vise, verify the direction you need to turn the pedal shaft to loosen it, and use a pipe wrench to take it out. Use a cheater bar if necessary, but you should be able to apply enough force and leverage this way to remove even the worst stuck part.