I wouldn’t necessarily suggest using an air impact gun…

Use a *long* breaker bar, and a *good*, snug fitting allen / hex socket (assuming yours is a 10mm allen bolt), and that should do the trick. They are VERY tight and use a weak threadlocker compound as they come from the factory. An old trick is to use some valve grinding compound on the working end of the allen / hex socket to help give it a bit more grip.

Sometimes the old adage applies: “when all else fails, use brute force & ignorance”