Hmmmm it's crazy, I've used hammers on spanners/sockets/breaker bars.
I've tried the old jump up and down relentlessly on breaker bar. All that did was move the car, even with the wheels chocked and handbrake blah blah blah.
My best looking option other than a rattle gun is to get another caliper from a wrecker, and angle grind the bolt/caliper off lol. But I'm not terribly confident that it would work 100%, and I'm not sure I wanna risk damaging other parts in there (the disc etc)