I have a 22"channel locks, and if that doesn't work I go to a pipe wrench, next is the sawzall and I cut diagonally across the top in several places, but you can forget all that with a 3' pipe wrench and a 4' length˙ pipe to make a breaker.
If you believe there was solder used you can apply some heat to the galv. nipple while attempting to turn the nipple with the breaker assembly.