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.
Your post made me chuckle quite a bit.
Believe me it's stuck proper and it'll take a lot more than a quick counterclockwise twist to loosen it. I know it's a large diameter, it has indeed some grooves on the sides (which gave me a nice, large blister on the inside of my thumb), but it won't budge.
Thanks for the warning Paradaz! I hope I don't damage it in the process... I've tried some more but it's still stuck.