What exactly could be a weak spot of a narcissistic individual?
If you are a regular person who lives life well and experience gratitude towards the world then you stand literally no chance of getting to a narcissist. You have a frame of reference which will never allow for you to get the best of them. This is why they say you should walk away and make your life a better life as punishment to them. The reality is you will never get the better end of them.
The reason is because of perspective. While everything is going great in the relationship and you're getting attached to the individual and they are at the same time really starting to understand your flaws and you are turning from ideal into disgusting.
While you are getting more and more emotionally involved and you are giving up more and more of your agency, they are building up walls and emotionally distancing themselves from you. The really hard part that makes this so devastating, is that they are acting exactly the same.
By the time that they are ready to walk away from you, you are fully invested and vulnerable. It's a sliding scale so to speak, where as you are getting invested they are disinvesting.
It's always this way, it will always be this way, and this is why you will always lose. You will lose because you are coming from a paradigm which says people want to work together in order to become a happy perfect couple. They are not coming from that paradigm they are coming from one where they want to be happy. They want to be happy, not they want to be a happy couple.
Everything comes second to them including you. This is the case right from the start. It’s just that in the beginning you are willing to tolerate the nasty things and look at those deeds like they are quirks. You tolerate these quirks because you think the relationship is going to turn into something better.
All that results in is somebody that slowly slices off pieces of themselves for somebody who doesn't want them. They don't want you, they want the perfect mate they thought you were.
You're never going to be perfect. Not like the narcissist.
You just won't do.