A plumber's tubular socket (of the right size) will go over the supply nipple and fit the nut after you remove the supply line. You can get them singly or in sets at most hardware stores.
I've also had some success with needle-nosed pliers after applying penetrating oil to loosen the grip of the plumber's putty and, but I had more patience then.

Here is a link that might be useful: Tubular Wrench Set

Thank you! My plumber also mentioned the tubular socket. (He can't come until tomorrow afternoon, which gives me a little more time to mess with this.) The nut is probably 1.5 - 1.75" diameter so I am not sure the sockets will fit around it, but I might go have a look at them and see. In the meantime, I have soaked it with more WD-40 and was able to unscrew it a little farther with my fingers (just enough to give false hope, probably, ha). So other than the fume headache I am getting from the WD-40, no harm done in me keeping on until the plumber arrives tomorrow. I'll try needle-nosed pliers too. The bigger wrenches won't fit in the tight space.

"The nut is probably 1.5 - 1.75"..."

Get a large enough socket.

You may need to go all the way to 3/4 inch drive sockets (or even 1 inch) to find one that large (and they are not cheap).

An import set should be good enough for a one shot job.

You can find a wrench like this that can be used by hand. I've seen them at both HD and Lowes.

"You can find a wrench like this that can be used by hand. I've seen them at both HD and Lowes."

The problem with the typical wrenches is that there is not enough room between the sink and the back wall to turn them, thus the use of large sockets.

The very large ring type nuts on many newer fixtures are much large than the typical thin/deep plumber's sockets.

I spent hours yesterday under the sink trying to remove the 20+ year old Hans Groehe faucet. there is a 3/16" thick black wedge spacer and a brass nut. i triend using a long plier and hammer to budge the knot but to no avail. i used a long plier to try and grab the nut to turn it but the angle wasnt good. i endedup removing the sprayer and handle to get at the brass threaded riser. i used a plier and turned it counter clockwise and finally removed it.