Hi, Im not sure what kind of narcissist you mean (just self obsessed and selfish? or plus abusive) and I don't have any definite answers but here's what Iv done being stuck with an abusive mum.
Ive gone to as many therapies, assertiveness courses and other confidence courses I can find and practiced being sure of myself and assertive. When she's on my back I speak slower and serious in order to make myself very clear - rather like I am telling off a naughty child. This has overtime been working one to one, most of the time, and feels to me like some sort of territory battle over my own brain. But she also uses really sneaky and vindictive tactics when it comes to the rest of my family and even my friends, in an attempt for control; it is incredibly frustrating but I cannot do anything about that. So I just have to keep an eye on her antics and focus on 'when I get out, she can simply get lost'.
So the best I can suggest is to join face to face groups where you can vent and discuss both her actions and your reactions too, and practice not letting her take control by pushing your buttons. I tried many different groups until I found one I was definitely comfortable with.
And believe in yourself that her problem is not yours - sorry there is no 'keeping the peace' with a narcissist.