get a cheap socket that is a little too small for the bolt head (you can use a metric or std. socket), put on some safety glasses, and hammer it on. Then use a ratchet or breaker bar to turn it. This worked like a charm for me in the past. Use a cheap socket as you probably won't be able to separate it from the damaged bolt.

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2005 Mercedes C230k sport sedan, 6 speed
1987 Porsche 924S - 968-engined track car