I have been removing those pesky nipples for years with rarely any problems. Simply drive a 1/2-inch round cold chisel into the nipple. This will stiffen the nipple and prevent collapsing and egg-shaping of the nipple. Then take a steel pipe wrench and put strain, “torque,” on the nipple with one hand in a counter-clockwise rotation. Then with the other hand, strike the end of the pipe wrench handle with a hammer.
This will almost always break any stubborn pipe nipple loose. I have taught many a plumber this technique. It really works great. Try it sometime.