Find a headset wrench to fit the lower adjustable portion of the headset and try turning it slightly clockwise to make sure it's not helping to bind the top nut. A high quality adjustable wrench - at least 12," preferably 16" will grip the top nut just fine. (Personally I have never been without a 16" adjustable wrench and a plastic dead-blow hammer. Headset wrenches tend to slip when applying extreme pressure. If that does not work carefully split the nut with a cutting wheel. You should eventually be able to pry the nut off in two pieces in a worst case scenario.