This achievement is for killing all enemies in every level. This is fairly straightforward if you play on easy, and level select allows you to hit any you missed. However, I do have a few tips.
1.) Go loud. There is no need to be stealthy, which will only slow you down. Additionally, every time a body is discovered, an exclamation mark will appear on your HUD for each enemy still alive in the current area. This allows you to have a general idea of how many enemies are left and where they are.
2.) If you change your skin, difficulty will revert back to medium. This threw me for a loop when I first went back to mop up one level.