If the nut is to spec, it should be torqued in the range of 200-270 ft/lbs so as Don said, will need a socket and long breaker bar to get it started unscrewing. Even when the nut is loose, if the drum has not been removed in many years it may also prove to be very difficult to break free of the axle taper.
Use of a large and proper style puller is imperative. You can probably find something to rent but it needs to be the size and type used on larger pickups and small trucks. Those used for the typical cars we are now used to will not be large enough. Even with the proper puller you may need to get it tight and let it sit pulling on the drum for a period. Come back from time to time and give the screw another hit with the hammer. Sometimes this can take hours. Above all, while the puller is attached and pulling have the nut loosened a few turns but leave it and the washer still threaded on the axle shaft so once the drum breaks free it can not pop off the axle. All that force suddenly released can send the drum flying and do real damage if unrestrained.
Here is a thread showing an older version of the proper type puller in use and if you do a search of the forum on brake drum removal there are other more modern versions of similar type pullers shown.
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