one-shot contest; make-or-break game; single-round match
A contest or situation where everything is decided by a single attempt, round, or decisive moment; no second chances. Often used figuratively for high-stakes, all-or-nothing scenarios.
The final is a one-shot contest, so I'm nervous.
This exam is make-or-break, so I have to prepare thoroughly.
一か八か describes any high-stakes all-or-nothing situation, whereas 一発勝負 specifically refers to betting everything on a single attempt or chance.
Compound of 一発 (one shot, one round) and 勝負 (contest, match). The exact historical derivation is uncertain, but the components transparently combine to mean a contest decided by a single decisive action.