He 'proved/showed his mettle' [=he proved/showed how tough he is] as a fighter tonight.
She'll have to be 'on her mettle' [=at her best] to win this competition.
The competition will 'test her mettle'.
(..) Our torment is near an end. Elizabeth. For years we've been tested and tried, and each man jack of you here has proved his mettle a hundred times over and a hundred times again! Suffered, I have! Punished, we were, the lot of us. (..)2003 Pirates of the Caribbean: The Curse of the Black Pearl