Are you trying to re-use the faucet?

If you clench your jaw just right, squint, and know the cheat code pattern of swear words, and have a decent pair of channel locks, you can grip onto that nut somewhat vertically and give it a slight turn. With the deck nuts in place (the side plastic nuts) it won't take much to release and then you can unwind it by hand. I've done it that way maybe 50 times.

If you're not going to re-use the faucet, then just grab onto the whole thing and rock it back and forth and it will eventually snap off. You can bend the threaded bolt without a nut on it out of the way to give yourself more space. Disconnecting all the supply lines and pulling them tightly to one side would also help.

Or, really deep sockets. Measure the depth of socket you need before buying any. Your standard deep sockets will likely not be deep enough. Husky makes a fancy "pass through" socket set with a hole through the socket and the extension. You'd have to know if it was metric or SAE though and make sure the sockets go down to a small enough size to remove that nut. If you're re-using the faucet you'll want the right tool to put it back on too.

If you're replacing the faucet, Moen uses a neat little system where you thread on a large nut until it's finger tight. On the face of that large nut are three screws that you tight to do the final bit of securing the faucet to the sink. It's super easy to do and makes working in that tight space very easy. Those skinny bolts with the long nuts are a terrible design.

Good luck.