has lost its inward points, this is what worked once for me: I hammered an Allen wrench socket which was just "light drive fit" into the bolt head and turned it out with that. Maybe I was just lucky, and maybe the driving helped by distorting the threads on one part or the other, but it did work.

Or are you talking about the kind of TORX bolts which have the TORX shape on the outside of the bolt head??