[ecomodded]

File or grind in a nick or groove on the edge of the nut and use the punch and hammer to spin it.

or

try heating around the threads with a soldering torch then try turning it

I often use a large or heavy hammer to smack the end of the wrench , in preparation file the worn nut or just hammer the next smallest socket on to it, then hit the end of the ratchet / wrench or breaker bar with a heavy hammer to loosing it.
If your still having trouble get a bigger hammer.