Take advantage, ask lots of questions, it's an opportunity to learn

Yes

Never talk with him/her again. And if I cannot avoid talking to them, I try to direct the conversation to topics they know nothing about.

Same way as any other person

Call them a retard and move on with your life

Honestly

Ask questions and listen

What kind of smarter? there's streetsmart, booksmart, IQ, EQ etc

I’ll let you know if I find one

Why can't I figure this out?

This is an incredibly vague question. What do you mean by "deal"?

If you mean some kind of unavoidable confrontation involving tactics and strategy then as in all such confrontations you want to be as aggressive and calm as you can, and of course perform to the best of your ability. There is much more to winning that just pure intellectual potential, you might not even necessarily be fighting an uphill battle depending on the composure and willpower of your opponent.

If you just mean general interaction and dealings, just use your best judgment to determine the morality of the person. If you find reasons to consider them untrustworthy, just leave. But if you're dealing with a decent person, they don't have to be your enemy, just because there is a hypothetical scenario where they can outmaneuver you in some sort of complex scheme.

I attack them with an army of stupid plebs,

jokes aside I stay humble, talk less and listen more <3