攻其不备
Take one off his guard.
To catch a weasel asleep.

eg：The sudden attack took the enemy napping and ran.

中英俚语俗语来源：孙子·计篇："攻其不备，出其不意。"

同义词：乘虚而入 Swoop down upon.
反义词：鸣鼓而攻之 Beat a charge.