Are you talking about the rear 36mm castle (axle) nut that holds on the drum? If so usually you would use an air impact & socket. If you don't have access to that you can get a special tool that you hit with a hammer to break it loose (remember it must goback on that tight too). They are widely available & inexpensive, here's just one of many examples;
http://www.bus-boys.com/tools.htm