I second the suggestion on Kroil. It is by far the best penetrating oil that I have used. Give it a little time to work then heat the caliper and then spray the bolt with something with something that evaporates quickly that will cool it. It is literally probably a crime to suggest it these days, but I made a fitting to spray from little automotive Freon cans. Use an impact wrench before the temperature between the bolt and the caliper get a chance to even out.
with the Freon I spray it with the can upside down so it is liquid when it hits the bolt. You need to wear gloves because it will cause frostbite burns if the liquid comes into contact with your skin.
Keyboard cleaner (duster) held upside down works as well.