If you can't drill it with the caliper still on the car (that does seem pretty hard), then just remove the rest of the bolts that are holding it to the car. IIRC you need to remove the two 14mm bolts holding the caliper bracket, the 14mm bolt holding the parking brake cable bracket in place, and the 12mm banjo bolt holding the brake line.
From there you can remove the whole caliper assembly with the rotor. Then you can work on getting that bolt loose/drilled on a bench.
Out of curiosity, have you tried soaking the bolt in penetrating oil for a couple hours? And have to tried attaching a short breaker bar to the bolt and beating it with a hammer? On my car the bolt you're talking about is the only one that came loose easily on my rusted-out, jammed caliper.