If the hex in the screw head isn't totally rounded out.......try this.

Preheat the bolt somewhat with a soldering iron. Use a precision ground 2.5 tool, put a dab of valve lapping compound on the tip of the tool, insert it into the bolt, turn slowly.

It's worked for me.

Otherwise a sharp left-handed drill bit. It often "bites" into the bolt causing left-handed turning of the bolt..........presto!