Club training with 心を鬼にする

Idiom Explanation: Taking a hardhearted stance with 心を鬼にする

Renge wedges herself into the Ouran High School Host club, appointing herself the club’s manager. Due to her social status, Tamaki and the other wealthy club members are unable to object. Instead, they leave it to Haruhi to oust her.

環(たまき):: 「これもホスト修行(しゅぎょう)だハルヒ！！」; "This, too, is host training, Haruhi!!"; 「おとーさんは心(こころ)を鬼(お)ににするぞ！！」; "Dad has to harden his heart!"