Finish a zone killing less than 50% of the enemies.

The most efficient way to get this achievement is to start the game at the last area of a zone (i.e. Area 4 is the last area of Zone 1). Start the fight off by using all of your nukes and grenades on the boss to weaken it up, then finish it off with concentrated fire, allthewhile taking care not to kill too many foot soldiers. This achievement can be unlocked on any difficulty.