Don't waste your time with heat, cold or penetrating oil. Stand astride the bike and set the crank forwards and horizontal. Fit a good quality pedal spanner to the pedal spindle so that it is also as close as possible to horizontal and parallel to the crank. Stand and if necessarily bounce gently on the end, being ready for it to let go. The horizontal crank and spanner will cancel each other out and all the force of your weight goes through the pedal spindle, which won't be able to resist. Be sure the crank is forward so that you are turning the spanner backwards.