Well if the impact gun wont take it off try putting some heat to it. Heat the nut...not the threads. That will sometimes break it loose.

What brand impact gun and how much pressure is the compessor set? Not to say you have shitty tools but not all impact guns are not created equal. Plus make shure the compressor is set to run at the guns recommended operating pressure. Most guns operate at 90 psi.

Good luck!