My process for this has never failed me when the fixed cup is the only thing left and is oldschool with the two flats for the wrench. Put the wrench on the cup, next have a small flat piece of wood or aluminum about 4 inches square, use a clamp, C style, ratchet bar, etc to clamp the square down on the wrench. This will keep the wrench from slipping off while you wrestle with it. I have an old Sugino with the hook on the other end that I can use a big box end with if I need more leverage. They are never as tight as they seem to be once you get the wrench to stay on. Like I said, this works for me 100% of the time. It will also work on other styles of fixed cups with the right setup.