YOU SAID:
he killed two birds with one rock.
INTO JAPANESE
彼は一本の岩で二羽の鳥を殺した。
BACK INTO ENGLISH
He killed two birds with a single rock.
INTO JAPANESE
彼は一枚の岩で二羽の鳥を殺した。
BACK INTO ENGLISH
He killed two birds with a single rock.
That didn't even make that much sense in English.