[TrumpyAl]

I'd try a single hex socket with an impact gun.

If it starts spinning once it loosens then I'd reach for multigrips to push the arm back 'home' and go again with the inpact gun so that it bites.

This almost always worked in my trade days with compressed air driven guns that had very high levels of instantaneous torque.

Nowadays I only have a 36v cordless impact gun, but I expect that it would still work. My gut feel is that the heat introduced into the male threaded part expands it so that it doesn't spin.