I'm not sure what the name of this nut is, but it has come loose. I have a "basin wrench ratchet head" that goes on my socket set ratchet with a long arm but it can't get purchase because it is too bulky. I have had some success with my thin vise grips but it eventually comes loose again and the nut is starting to get scratched up from it all.
Since you'll not likely need to use the tool often, you might try an open-end wrench held along the supply lines, if you have one on hand of the correct size. It won't allow much torque before slipping, but it might suffice.
Both of these answers were great but not perfect for my circumstances. Today I had to replace the cartridge in my faucet, and in the process had to loosen this nut and tighten it once the work was done. Here is what I used -- as an answer to my question:
I used vice grips with their nose clamped onto the nut perpendicular -- so pointing straight up around the nut.
I had a helper use a non-marring strap wrench to hold the faucet
straight at the top.
I tightened the nut from below using the vice grips.
Once hand tightened, I put a 12" socket extension between the vice
grip jaws and turned a 1/4 turn to do the final tighten.
Maybe this answer is helpful to someone else in the future.