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'.
(..) did forfeit with his life all those his lands... ...which he stood seized of to the conqueror Now sir, young Fortinbras, of unimproved mettle hot and full... ...hath in the skins of Norway here and there sharked up a troop of lawless resolutes For to recover of us by strong hand (..)2015 Hamlet