trial by fire

noun3

1. •
   A test in which a person is exposed to flames in order to assess their truthfulness, commitment, courage etc.
2. •
   A situation in which a soldier or other combatant faces the discharge of opposing weapons, as a test of their fortitude.

   • French troops, submitting to a trial by fire, drew toward the German forts, capturing and holding some machine-gun nests. — 1939
3. •
   Any ordeal which tests one's strength, endurance or resolve.

   • But take it all in all, the overland trail was a trial by fire. One gets a notion of its deadliness from the fact that over five thousand people died of cholera alone. — 1918