For one of my axle nuts I threw impact guns and all sorts of breaker bar contraptions at it with no luck.

What finally got it was me jumping up and down with full force on the end of about 6' worth of 'breaker bar' (read pipe). That BARELY did it.

That's about (guessing I was probably ~180lb at the time)...1080FtLb of torque. Your impact gun probably ain't gonna do the trick