Before you get vicious with it get some Justice Bros.'s, JB80, like the can says "Twice as good as the other stuff" really.
Spray the nut down and take a long punch or screwdriver and give it a sharp rap. Not enough to bend metal, don't bugger-up the threads.
Then come back a hour or two later and give it a try.
Best stuff I've ever used, better than liquid wrench and wd40.
Oh yeah, if you have tried wd40, squirt it off with some breake cleaner, then use the JB80.