What about an air powered palm hammer normally used for hammering in confined spaces. Mine has got me out of trouble a couple of times. It uses several percussive hits per second. I've knocked out large rivet centres with ease as well as knocking back the metal bracket onto the base of the side window glass of my car, where a regular hammer wouldn't.
Worth a try on this.

Ahh, if only victa had kept producing the thumblatch catcher series, they would be in better shape today!