That big nut can get VERY stuck over time! I do recall it being a standard thread. Soaking it with PB Blaster or another good penetrating oil is a HUGE help.

I made my own version of the factory tool - it is basically a "pitchfork" where the legs fit around the 4 bolts on the front of the fan. If you have it out now, maybe you can clamp those bolts in a vise?

"Professionals" very frequently use an pnuematic hammer with a chisel bit. Set the chisel near the peak of the hex and hammer at an angle. Makes a mess of the nut, of course, but will break them loose very quickly.