trial by fire

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A test in which a person is exposed to flames in order to assess his/her truthfulness, commitment, courage, etc.
• noun A situation in which a soldier or other combatant faces the discharge of opposing weapons, as a test of his or her fortitude.
• noun idiomatic, by extension Any ordeal which tests one's strength, endurance, or resolve.