You need a pipe wrench. Try to borrow-they are pricey.
A basin wrench is a necessity for bathroom installations involving news basins, sinks, and faucets. To install a faucet, find a basin wrench at your local hardware or grocery store.
A Basin Wrench
A basin wrench is a tool to remove and replace the nuts that hold a faucet in place. It works in tight, low visibility places for sink repair. The basin wrench looks like a long steel bar with a claw on one end. This claw is usually spring loaded to keep a tight grip of the nut being turned, and can 'flip' to enable turning either way easily. The other end makes a "T" as a smaller bar goes across it.
A wrench is a tool used to turn a bolt or nut.
A 35mm wrench....
With a basin wrench
An anchor nut is a nut designed not to require a wrench for tightening and loosening.
A 14 mm wrench.
An open end wrench.
You can use a crescent wrench, or pipe wrench. Both are adjustable to fit the nut you are working on.
I have removed many with this procedure get a good quality socket slightly smaller than the locking nut hammer it on to the locking nut but no to split it that's why you need a good quality socket, Chrome Vandium spec, then attach the wrench to undo the locking nut.CozyMerc.