When all else fails heat the nut or bolt with a propane torch then apply candle wax. The wax wicks down the threads and lubes them.
Anothr method if you can get at the nut is to use a sharp cold chisel and hammer. A couple of hits on the flat of the nut will crack it and it will come off easy. You didnt want to re-use it anyway. When i was an apprentice my old boss would say. " dont worry about your hand when cracking those nuts off. After you hit it the second time you wont any more after that." He would then walk away chuckling to himself. He was right!!