at a blow
  一擊,一舉,一下子
  Blow, n.
  1. A forcible stroke with the hand, fist, or some instrument, as a rod, a club, an ax, or a sword.
     Well struck ! there was blow for blow.   --Shak.
  2. A sudden or forcible act or effort; an assault.
     A vigorous blow might win [Hanno's camp].   --T. Arnold.
  3. The infliction of evil; a sudden calamity; something which produces mental, physical, or financial suffering or loss (esp. when sudden); a buffet.
     A most poor man, made tame to fortune's blows.   --Shak.
  At a blow, suddenly; at one effort; by a single vigorous act. “They lose a province at a blow.” --Dryden.
  To come to blows, to engage in combat; to fight; -- said of individuals, armies, and nations.
  Syn: -- Stroke; knock; shock; misfortune.