I had a big breaker bar with a 1/2" fitting on the end that I used without any attachments to remove this plug (if it is the drain plug you are referring to. I had great leverage it with it and it wasn't a problem.
On the filler plug I used a big crecent wrench (didn't have anything better at the time) and that worked as well.
Hopefully that helps although it probably doesn't! You've probably tried those items .