I used a Sawzall. The same thing happened on my Spirit when I did the rear shocks. Rounded off the head and couldn't get anything in behind the bracket. I cut up the rubber isolator with a utility knife and pulled as much of it off as I could. Then, I just cut straight through the sleeve and bolt on both sides of the shock. Don't use an angle grinder, all those sparks could easily start a fire. Get a pack of hard metal cutting blades and pull the blade out of the cut every 15-30 seconds to wet it down with some WD-40. It takes a while but you will get through. Even if you could get a hammer in there, I doubt you could drive that stump out. I'd be willing to bet the reason the head broke in the first place was heavy corrosion between the bolt and the sleeve, which essentially welds them together.