My understanding is it's generally un-curable since it's such a deeply rooted and ingrained coping mechanism for the ego. Like an addiction. But if I had to guess, someone would probably have to deeply work on true self-compassion (a lot of narcissists are, internally, extremely self-critical, if I understand the condition correctly) and build back from near zero. Sounds really difficult.

A heroic dose of shrooms might be one vital component of a solution.

It's intractable. There is no solution except teaching people how to identify and protect themselves from narcissists, or institutionalization.

People with narcissistic traits can work towards undoing the damage done to them; but, once you're at the level of a personality disorder it becomes more about coping with the inevitable and continuous fallout. Genetics, trauma, and childhood abuse all come together to create a shell of a human being who, motivated by self-preservation, exists only to consume, exhaust, and expel other human beings.

There are no evidenced treatments, and there is no saving them. They are not capable of changing -- if they were, they wouldn't be (by definition) narcissists. The best thing we can do is teach people to identify and protect themselves from these lost souls.