I agree that everything Augie said is "spot on".

If possible, spray on some penetrating lube the day before you are going to knock off the frozen nuts. Give it time to work, and then spray again when you are ready to start working.

I would add - also try turning them the wrong way. You won't be tightening the nut. Usually, the problem is the nut is frozen, rather than too tight. Once you get it moving in either direction, it will be much easier to remove.

Finally, "shock" impact is much more effective than continuous torque. I have seen a crew put two guys on a cheater pipe and not budge the nut on a 2" bolt. They then hit it with a 3# sledge hammer and it comes right off with only one guy on the cheater. (This is the reason an impact driver/drill is so much more effective.) Obviously, you can't hammer harder than the bending strength of your bolt.