I would have used a 6 point socket from the start not a 12 point now that it's all fudgeed up I would get a big pair of channel locks and use a manly grip that's about all I can suggest .
Not sure if heating it up will help .
If this dont work dremel slot the middle and use a heavy duty screw driver that you can put a Cresent on the shaft to turn it or a pipe wrench .
grind the heck out of it on one side until you almost see the threads. hit it with a chissel and a hammer counterclockwise on the grounded side. that baby will come loose faster than I fart. heck, I have even remove them by hand after grinding