Breaker bar!!! Dont care how good of an impact gun you have leverage always wins. If and WHEN i run into this prob the 3/4 24" breaker bar comes out and put a 3' pipe on it and i have "yet" to be stopped!!
^^I'm a fan of Kroil (get it for free from work ) and a big breaker bar with a snipe. Remember, you don't shock the bolt, but constant pressure and penetrating lubricant will move almost anything, except an ex-wife's sex drive.