Help! Foolish me playing mechanically inclined again.

If you have a small pipe wrench available and enough clearance you might try that. Has worked for me in the past. It sounds like the nut is damaged anyway so no worries there. The jaws of the pipe wrench will clamp down on the nut and won't allow it to slip like vice grips do. Just a thought. Worst case scenario the bolt will twist in two. Tim