heat has always been my friend when it comes to rusted up stuck bolts, get the nut side nice and red, and it should come out, i do the tighten it first and then back it out,( work it back and forth some) just to clear up the threads and then crank it out.

a plumbers soldering torch works just fine too, just takes longer to get it good and hot. you can pick them up for under 20 at home depot.