This is by Adriana McGee from quora. She answers the question:

How do you defeat a narcissist?

There are actually several steps to “defeating,” or getting your power back from an abusive Narcissist, of which the first one is the most powerful and essential:

You confront them with the enormity of all of their misdeeds and tell them that you refuse to accept their abuse anymore.

You disengage or cut ties with anyone you know has been influenced by their smear-campaign, at least for the time-being.

If their actions are criminal, you document and report everything.

You forgive yourself, and cut any emotional, material, or financial ties to the Narcissist.

You protect your future sanity by employing the ethics of “no contact,” by “icing” them, or giving them the “silent treatment” when necessary.

You accept the fact that they are normally very weak, insecure, and unhappy people, whom are bound to destroy themselves in one way or another; you isolate the threat by ignoring it rather than destroying it.

It is very cliche; but you “take revenge” on the Narcissist by living a happy and fulfilling life. Not to prove anything to them, but to prove to yourself how much stronger, happier, and more content you are without them. You don't allow them to make you second-guess yourself, or allow them to destroy your self-worth, self-confidence, or self-esteem; you take care of yourself because that is what is best for you.