YOU SAID:

There's no way he did it by himself.

INTO JAPANESE

彼が一人でそれをやったはずがない。

BACK INTO ENGLISH

There's no way he could have done it alone.

INTO JAPANESE

彼が一人でそれを成し遂げることは絶対にできなかった。

BACK INTO ENGLISH

There's no way he could have done it alone.