I like PB Blaster or Kroil. And a piece of pipe slipped over the end of the wrench for more leverage. Think about where everything is going to impact if it comes loose suddenly. Will the handle hit you in the jaw? Will your hands slam into a cabinet brace? Adjust accordingly. You have your tetanus booster up to date, right? And don’t forget to wear these…tap, tap…safety glasses.
Agreed with above, only brute force and a little penetrating oil will do.
A lot of times I’ve pulled the whole damned sink out of the hole just to get the leverage, believe it or not it’s turned out easier and less effort in the long run that way.