In what episode did Karin meet Orochimaru? There is an episode in Naruto series when Orochimaru protected Karin against some guy.

Which episode is it?
It's from Naruto Shippuuden  Episode 431: To See That Smile, Just One More Time.

This one is from the run of Infinite Tsukuyomi fillers that aired during the Fourth Shinobi World War arc. Notably, unlike the previous episodes in this set of fillers (mainly just fairly lighthearted happenings inside the Infinite Tsukuyomi dream of various characters), Karin's episode here is basically a rather sombre flashback to her past.

(This was a sad episode. Poor Karin...)