You stripped the head of the hex bolt?
If there is room I would try an EZ Out. Take another like bolt to a tool store. Have them match the EZ Out to it. The $64,000 question is to check and see if the EZout will have enough room.
I have been there ... I now use some sort of a flush to clean out debris out of the head. I also whack the allen socket securely making sure it is seated before starting to back 'em out. I just showed this to my son and daughter the other day while working on the car with them.