This means that you will have to complete a mission by only using Emplacements to kill the enemies. It is recommended to wait until you have the top tier emplacements before attempting this achievement.

Alternatively, any legs that have the 'Smash' ability can be used to attack enemies.

The easiest mission to do this on is the first mission of the first act 'Europe' called 'Beach'. As the achievement describes, you will need to do this in single player to unlock the achievement.

~Int