I'm pressuming you have tried a vise grip or a wrench and these havn't worked. So the only other option I can think of is, using a hammer and chisel and strike the bolt to make a groove, then just keep hitting the groove in the direction of the thread and it should turn.
File some flats on it or cut a slot so you can get a grip with something. Use combination of heat, plus gas, and shock to loosen it. Try tightening it a little before loosening it.