I've been working PDN since January, and finally 'landed' a day job where I work with just one patient. The night nurse (we both work 12 hour shifts) is a 'lite' version of your 'adversary' lol. I too am not aggro or confrontational, but over the years I've developed my own 'style' of being assertive, direct and honest with difficult co-workers. There is not just one 'right way' to deal with difficult co-workers -- there are hundreds of effective ways, not that I can provide a list. It's the idea that you can (and will) develop your own personal way to deal with nurses like this. I've been a nurse a long time, and I'm telling you, there is 'one' wherever you go.
The night nurse (an LPN) has been with the patient for years as well. I worked part time with this patient since January, and full time since June, so I 'granted' her the "O Significant One" title because she's been with him so long and knows him so well. She is also a classic Chart Nazi, and can find a missing comma or missing set of initials with her eyes closed.
She is also generous with her 'why we need to do it this way', and it always concludes with "what if the state/auditors/God Himself were to see that you put 240cc for his tube feeding instead of 237cc, like it says in the orders?" Every day, there is something. The day before, it was "I know this is silly, but could you please fold the end of the tape over when you take off a piece, I hate to spend all that time hunting for the loose end" and yesterday, she found the lid of a saline bullet in his bed and 'just wanted to remind me to make sure I clean up my garbage'. These included 'explanations' as well .
The patient's mom intercepted a two page 'love note' she wrote me when it was discovered that a doctor's order to increase the nightly melatonin got lost in the chaos that is our local agency office. It wasn't her fault, or mine, or anyone but the nurse manager who quit work that same day and probably threw the new orders out her car window in her mad dash to escape her job, but she is so insecure and terrified of 'getting in trouble' that she threw the patient's mom under the bus already, and was trying to stuff me under there, too. The mom chewed her out for the 'love note' and I never saw it lol.
Anyway, how I deal with her is to deliberately consider what she says, and then 'file' it. When she's right, she's right, and I tell her. When she is OCDing due to her insecurity, I give her my whole attention and do active listening. It shuts her right up . Fortunately for me, her insecurity is so enormous she backs down.
To some degree I understand her protectiveness, and forgive her for that because I am rapidly becoming close and protective of the little guy too. I get that. But I wasn't born to help her manage her anxiety disorder, which often comes disguised as being demanding, nit-picky and critical.
You have every right to do your job as you best see fit, and to continue to do so regardless of her commentary. I have crumpled quite a few 'love notes' myself. Not my problem. If she has a point, that's fine. When she doesn't, not my problem. What you do is emotionally detach from her. She doesn't have YOUR permission to criticize or judge you unnecessarily -- and you decide where that line is.
You don't have to get into a whoop-n-holler cat fight to set boundaries with her. Do what comes naturally, try different things, and -- relax. She doesn't have your permission, remember? Blah blah blah blah. Crumple the 'love notes' . . . and compliment her. I've found that to be very effective with the night nurse. I don't mean make stuff up, but genuine realistic compliments. Usually people who are hard on their co-workers are compensating for insecurity. It may be the last thing ya want to do, and it may shock her at first, but if you are attached to your patient and want to keep working with him (as I do), whatever you can do to help her TRUST you (another major deficit with difficult people).
It's not like I don't have my . . . revenge, though. She has these 'special butt pillows' and the chair I sit in his HER special chair she brought in herself. Yesterday, I had bad gas. Sorry!