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.
What is the correct tool for this job?