From what I have learned in dealing with our psychopath child, here is the Achilles heel or kryptonite:
1. They can't keep their lies straight which exposes their games
2. Those who won't play their game frustrate them. They can't handle when they can't hurt others or cause strife-drives them batty.
3. They can't distinguish between different emotions that are not direct opposites. They can tell the difference between anger and happy, but they can't figure out certain emotions like pensive and sad or happy and exhilarated (the gray areas). So, you can have the upper hand in using this as kryptonite.
4. They can only read faces to know how to manipulate. Take away the face and then they can't figure out how to manipulate.
5. Their computer goes blank if you ask them to explain further a statement they make of remorse, sad, happy, etc. They only know to repeat words, and they don't know what any emotion feels like because they have never had any.
6. They do not understand irony or sarcasm. If they do something to cause harm and you give them something as sarcasm or irony with a laugh, their computer goes blank. They lose control.
7. Achilles heel or kryptonite is when they can't bring harm to others or cause strife, triangulation, etc. Limiting number of contacts with only those contacts knowing how to respond to frustrate the games would be kryptonite.
8. Throwing the game back at them makes the computer go blank.
9. Okay, biggest kryptonite: HIDDEN CAMERAS. This particular insanity likes to hide like a cockroach in the light. They want everything done in secret so they can be in control. If they are being watched, they will do all they can to be sure to hide what game they are playing.
10. They hate it when they can't control the emotions of others or an audience (they play to audiences). They will try to act sad and pitiful with a room of happy people. If you laugh at their sulking face, they are mad that they can't get you to feel sorry for them. If everyone is upset about something, then they will act happy (and if you ignore their happy by walking away they get angry). They hate it when they can't change the atmosphere with their fake emotions/moods.
11. Their games are recycled. The right hand hemisphere of creativity is NOT there. So, they play the same games over and over and over again. If you keep a journal then you can better spot the next game (have a plan ready).
I think most of them if not all apply also to an adult Psychopath, at least they fit to mine, especially points 2, 3, 5 (a big one) and 11. I'd also add that it's easy to spot when they're about to target someone when they switch masks/moods from moment to moment when a given person comes into a room. Hidden cameras are good but also voice recordings - the Psychopaths often make covert threats, lie and gaslight and they feel you a word salad - this is not only good evidence but also sanity check (if you can go back to it later you actually can pinpoint the inconsistencies and manipulations which will go unnoticed when talking).
Another point I'd add:
Psychopaths have real problems understanding and creating metaphors, especially those which have to do with emotions. If you talk metaphors to them they have no clue unless they are simple and well rooted in the language - agin you get the blank screen. I've used to laugh on my Psychopath that he doesn't understand things like that and therefore completely misses most of the art (on top of it he didn't dream) and later on saw a research paper which suggested exactly that.