It took me a very long time to see a pattern of convert narcissism and toxicity in one of my friends.

Had I caught this soon enough, I would not be in a predictament that I am in now. Because I used to be very close to her and then started distancing myself as I saw her true color, she is now questioning me why.

I don't know how to respond, as I know that her response to whatever I say would not be sincere.

A while ago, I set a boundary with her a little more explicitly when I saw that she continues to push boundaries when I try to set them gently and implicitly.

She acted as if she understood by saying all the right words (she tends to exaggerates in a way that is too saintly/ angelic to be true). Then she followed these words with passive-aggressive behavior that clearly showed she was offended. I have also seen her do the same to someone else.

How do I set a firm boundary with her, when I know that she will only pretend to understand?

You don’t. People who continuously cross boundaries are typically toxic. You yourself say she’s toxic, so why continue being friends? I would completely distance myself and be around healthier people.

[MickeyCheeky]

I agree with golden_eve. If you don't like being around her, I'd suggest to just cut off contacts with her.

I have found that when you set boundaries, some people will really test them and push you pretty hard. It’s a toxic behavior. It’s not respectful. I had to learn to stand my ground. It’s hard to do if you haven’t always done that but well worth investing the effort in yourself.

[eskielover]

I would also just tell her you have become involved with things that are consuming your time & energy & just aren't available or able to keep that friendship active. You can still get together once in awhile but just keep distincing more & more until it just fades away.

Yes, standing up for boundaries is important to learn BUT the amount of energy required to enforce boundaries with a person like that may not be worth it for you. It could be way too exhausting