Air tools. If that's not an option, a very long bar is always the way to go. As long as you have a good 6-point socket, I'd not be afraid to put a lot of pressure on it.
If you're worried about breaking something, try heat: use a torch to get the center bolt hot - red hot if you can. The heat will burn out crud and the expansion will loosen things up. I imagine the rubber will burn and smoke, but it's junk anyway now.
