to heck with the hammer routine for that ring. Go down to your local good hardware store and buy a metal plumbers wrench for a 2-1/2 inch nut. Cut off the tips of the wrench so you have square ends. you will find that the wrench is a perfect fit to undo that ring (place it verticaly, it has a deep enough throat that it will clear all the wires and pipes). Use a pair of vise grips to hold the top of the wrench and turn it. Usually, those wrenchs are double ended. This allows you to put a cheater bar on the end and get good leverage.