Genius narcissist that can convince anyone of anything

I do family law, so I assume I get this more than other areas, and I usually have at least one case with this at all times.

One party states that the other party is just super smart, maybe a genius. They are also a horrible narcissistic person. But they are just so charming when they want to be, that they convince everyone that they are a great person. And maybe they a POS, not really my point. Usually, both people are at least kinda shitty.

But every time I have dealt with a case with this charming genius, the person is never that smart. It's usually just that the person making the allegation is dumb.

I need to figure out a way to have that conversation with my client. "He isn't smart, you are just dumb. So if you are having a battle of wits with them, you REALLY need to check in with me before taking action." I have tried dancing around that conversation, but they never listen. Usually because they are dumb.