When a bolt or screw is stripped, I drill it out with this extractor set.
When working on old motorcycles, or just my old VW bus, I encounter a lot of stuck screws and bolts. Frequently prior mechanics have left fastners completely rounded off. When these things happen, I grab my drill.
These extractor bits both burnish the stripped head, and then screw into it and extract it. They are not simple to use, and it takes a very steady hand -- something I don't always have -- but slow and patient work generally produces good results.