Whats the best way to get this fitting off

Post #6 is the closest way that I would do it. Have done this several times. With a die grinder type cut off, cut the nipple off about 1/8" from the manifold. Then with another die grinder and metal cutting bit, go inside the nipple and start cutting a groove. Be careful not to go into the threads, when you get close, take a small chisel and start caving in the nipple by the groove. If you don't have the equipment, you can try the other ways. I wouldn't want to wait that long, I could have that out of there in 5 minutes.