Couple of options...

Two man mode...get a long pry bar and pry up onto the bottom of the arm and the other person try to tighten the nut.

One man mode option one...heat it slightly, and try an air impact wrench, or try and move quick with a regular ratchet.

One man mode two....Get a large C-Clamp, Clamp the bottom of the joint to the top of the knuckle, tighten real tight, try to tighten nut.

One of these options will work. However, being frustrated does not help. Drink a beer, smoke a joint, do something to calm down, then go back to work :thumbsup