Hrm... a quick counterclockwise twist was all that was needed to loosen mine. I'm surprised you're having any issue at all, hot/humid environment included.

The nut is fairly large diameter, the threads aren't that fine of a pitch, and the nut edge is heavily knurled (has ridges) so you should be able to get a lot of torque on it to free it up without damaging anything, as long as you're turning it the right ...er, left way.