What does “deixa de ser se achão” mean? (a typo maybe?)

Vedi una traduzione

The answer above is actually pretty good, but I'm going to add my two cents...

In Portuguese, sometimes you can use the augmentative case to make up noun versions of verbs with the meaning of “someone who performs the given action excessively”.

For example:
- fugir (to run away) -> fujão (a runaway)

- apelar (in the context of gaming, it means “sweating”) -> apelão (a sweat as in a “try-hard player”)

- causar (slang for messing around) -> causão (a troll / a person who messes around a lot)

- achar-se (to brag) -> se achão (a brag)

So:
“Deixe de ser se achão”
“Stop being such a brag”