First, identify the narcissist's weaknesses (there are usually plenty to choose from). Is she vain about her looks? Money? Education? Whatever it is that she considers herself to be the most amazing at, that's the weakness to target.
Then, systematically destroy the image she's built of herself. If she thinks she's the hottest thing that's ever walked on two legs, either make yourself over, or find a very attractive, friendly, sweet natured chick to hang out with. Constantly refer to how hot other people are. Watch her squirm.
If she's hung up on her intelligence, even better-intelligent people really are a dime a dozen. Again, praise others-but never the narcissist! The more people you can get to do the same, the better.
Eventually, the narcissist will snap. They can't stand to be ignored, and they HATE when other people get more attention than they do, so they'll do something stupid and revealing. Then everybody will see them as the monster they really are, and they'll have to move on to greener pastures to get their attention and praise. Expect to be attacked, vilified, and vandalized in the process, though. Narcissists really are evil, and they're too stupid to give up.
Narcissists tend to be bad losers and they tend to want attention, so either beat them or ignore them.