O.k... I just checked my pictures from when I did my rear brakes. Bolts on the rear brakes can be a real issue. If you haven't already stripped off the head of the bolt, you need to get leverage...leverage..leverage. Either you'll twist the bolt off (then you can just get the broken part drilled out) or it will come loose.
Lefty loosey, Righty Tighty. And that's with you looking at the head of the bolt.
Next, you need a 1/2" 6point socket and a 15" breaker bar.
Then if you need to get even more serious with it pickup a 2 foot piece of conduit from the hardware store.
The soak the heck out of the bolt with a liquid wrench type penetrating oil. Smack it with a hammer a bit to help the fluid migrate into the threads. Let it sit for a couple hours.
Then try to get it with the 15" breaker bar. If that's not enough, cut a piece of conduit short enough to fit in the space. You may need to pound one end to oval it out so it can fit on the end of the wrench. Then slip it on the end of the wrench and work the bolt some more. If a 15" breaker bar, plus 8" of conduit doesn't break it loose, or twist it off, you're hurting.
You might grab a friend and have them heat the edge of the bracket while you tap on the end of the breaker bar with a 3 lb. sledge. But really the 15" bar alone shoudl be sufficient.
You must have 1/2" and 6point otherwise the socket or the wrench could break.