HSK 7-9
埋头苦干
máitóu-kǔgànto bury oneself in work (idiom); to be engrossed in work; to make an all-out effort; up to the neck in work
团队埋头苦干终于完成任务。
Tuánduì máitóukǔgàn zhōngyú wánchéng rènwù.
The team worked tirelessly and finally completed the task.
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Appears across levels (7-9) in the HSK 3.0 word list.