That nut is TIGHT and of the many times I've had to take it off, the best tool for the job that works consistently to make it a fairly easy job is a big strap wrench to hold the fan pulley.
Take the serpentine belt off. Position the strap wrench so it gets tight as you are pushing on it towards the driver's side while you pull towards the passenger side on the big wrench on the nut. I use a huge crescent wrench on it and sometimes clamp a set of vise grips onto it and pull from the vise grips for more leverage. That pops the nut loose every time with the least amount of drama.
If you see two different sizes of strap wrench, make sure to get the bigger of the two. The strap of a quality strap wrench will be multi-layered like a serpentine belt. Look at the side of the strap to verify you see some threads in the middle of iit indicating multi-layer. The strap on cheap strap wrenches like Harbor Freight Tools sells are solid rubber which will snap without much force being placed on it.