To be a tough nut to crack / To be a hard person to deal with
Literally 'to be a hard bone to chew'. Describes someone or something very difficult to handle, convince, or overcome.
The director is a tough nut to crack in negotiations.
—How did convincing him go? —He's a tough nut to crack, but we managed.
That exam is a tough nut to crack. You have to study hard.