Sometimes we suffer from some one else who did not understand at another time and over tightens the pedal in an attempt to get it off.

Take the crank off the bike and carefully put it in a padded vise. Heat the pedal end very carefully just a little with a weak torch, then quickly before it cools too much, and using a breaker bar on the pedal wrench take off the pedal. If the pedal wrench is damaged get a cheap slightly smaller wrench and slowly grind it open to a tight fit.

Have you already put some Liquid Wrench on the threads, tapped the pedal for a while to vibrate the parts and let them soak over night?

When it's time to put the pedals back on use anti sieze instead of grease. Make sure to get some on all the threads all the way around, then wipe off the excess before installation. After your first couple of rides ride check the pedals again to see if they are still tight.