HSK 7-9
精益求精
jīngyìqiújīng(idiom) to constantly seek to improve; to keep perfecting
师傅对木雕技艺精益求精。
Shī fu duì mù diāo jì yì jīng yì qiú jīng.
The craftsman continually seeks to perfect his woodcarving.
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Appears across levels (7-9) in the HSK 3.0 word list.