The best way to make a narcissist weak is by standing up for yourself and enforcing healthy boundaries. That means you refuse to engage in any power struggle with the narcissist.  Instead, you take matters into your own hands and set healthy limits for yourself. It will ultimately make them feel less in control and weaker. Additionally, make sure the narcissist understands that their behavior has consequences. Let them know in no uncertain terms that their actions have negative consequences for you and them.  Make it clear that you're not going to play their game and that you're not afraid to stand up for yourself. Lastly, show strength by refusing to bow down to their demands or manipulations. Don't let the narcissist manipulate or control you in any way, shape, or form. Stand your ground and be confident in who you are.