en_tn_condensed/act/18/17.md

So they all seized Sosthenes, the ruler of the synagogue, and beat him in front of the judgment seat

Possible meanings are 1) the Gentiles beat Sosthenes in the court in front of the judgment seat because he was the Jewish leader or 2) Sosthenes was a believer in Christ, so even though he was "the ruler of the synagogue," the Jews beat him in front of the court.

they all seized

"many people seized" or "many of them grabbed"

Sosthenes, the ruler of the synagogue

Sosthenes was "the ruler of the synagogue" at Corinth.

beat him

"repeatedly hit him" or "repeatedly punched him."