Does the ring on the outside spin or something? if that's the problem and you don't want to put it back on something like a vise or large c-clam could be used to crush the ring until it bit into the rest of the fitting and could then be loosened that way.

Also that link says it's a left handed thread which means righty loosely.

Tldr: getting it off shouldn't be too hard if you don't mind breaking it. Murph's tool was designed to not damage the fitting.