Rival
If this is a new Pietta and you have never taken the nipples out, then you might meet the Pietta gorilla.
He installs the nipples. You can search this forum for a lot of advice like that on BlackPowderBen's post "New Black Powder Pistol" from September 25th this year.
As others have suggested, get a
good nipple wrench. There several styles. This is where the experience of others might prevent you from needing to buy another one later.
I know I prefer a 'T' handle as it helps me to keep the wrench securely on the nipple (cone).
One of the best things I learned here was steady and sustained pressure on the wrench.
I haven't tried Kroil yet, but it sounds good if nothing else that you have works.
Also a rounded off nipple can be a pain...take your time.
I purchased a couple 'unfired' pistols that had rusty gunk above the nipple threads under the nipple seating shoulder, so I think it's good to remove and clean them.