How to remove retainer nut without scratching the finish

[algal27]

Hello,
Please forgive me ahead of time if I don't use the correct terminology, but does teach me the correct word. I am not a plumber.
I have a very expensive kitchen faucet that drips from the single handle. There is no brand that I can see to look it up that way. I beleiveni need to replace the cartridge but I can't get the retaining nut off. Can someone give me any pointers? Please see pictures.

Thank you
Allison

 

[Geofd]

The bottom right picture, it looks like that price over the cartridge may thread off

 

[GReynolds929]

Hands, strap wrench, or plastic jaws on your channies. That looks like a Moen, they should send parts for free if you contact them.