Unless it's a hydraulic nipple a pipe wrench has a real good chance of only egging the nipple. penetrating oil - don't waste your time. As previously mentioned, cut it off flush and then split it by sawing. If you are an X-pert with a saws all go at it. If not clamp a hack saw blade between a couple of strips of wood and cut on the pull stroke. You'll be able to feel when the teeth first contact a thread. If the length isn't cut off, one cut often does the trick using a couple of hammers, 1 as backup. Cut off, 2 cuts may be necessary.
Not many plumbers have a Proto easy-out that size.