Astro Pneumatic Flabk Bite sockets (and box end wrenches) work well within their limits.
A shake-and-brake tool (Mayhew is one reputable company that makes them) on an impact hammer while you apply torque with a box or open end Wrench is another possibility. Or just a box end Wrench and the air hammer with a flat punch tapping on the bolt head while you apply torque. Sometimes the shaking vibration gets things moving just enough.
But it sounds like the head is rounding because the bolt is stuck. (Captain Obvious strikes again
). Penetrating oil or heat is usually needed to help those ones. Not at the same time unless you are looking for smoke and flames. Shop made acetone and ATF mixture if you don't have anything 'better' (but I usually use Kroil, works pretty well).
That and some caliper bolts have red(ish) threadlocker one them from the factory. Red as in heat (500F or so) is needed to get the threadlock to let go. Pin point flame from an oxy-fuel torch usually works better than the broader flame from a plumber type air-propane torch.
Or go to the weld a nut on the existing bolt hex head approach. Gets you a bigger diameter to apply torque to and an intense focused heat during the welding. Sometimes it takes a few tries but it never failed me yet (crossfingers) removing a broken stud/bolt/screw when the head already snapped off because the threads were stuck.
That and try the approach of TIGHTEN a little and then loosen a little, back and forth. Sometimes that can get the threads to start moving.
