Tricky because of it's location.
Give it a good soaking with penetrating oil twice a day for a few days and the try to fit the next size up allen key or drive a suitably sized torx head into it.
If you're careful, you may be able to file the head off it and when you remove the caliper, it should be possible to screw what is left of the nut in a vice and a bike shop should be able to give you a replacement nut.
Alternatively, cut through it from the other (caliper) side with a hacksaw. Obviously this will bugger up the caliper mounting bolt so the caliper is probably toast unless you can get spare parts.