Get a total of 50 Trap kills while playing the story mode in either of the two stories. To set a trap just walk up to a tree and press and and hold the RB button until the branch is fully set. Set the trap in a good spot, preferably near a lot of mercenaries and throw a rock nearby. Wait until you hear a snapping sound and that means you killed the merc.
Note: For this and all other single player achievements requiring 'X number of kills' of a certain time, the kills are cumulative across multiple plays through. If you beat the game on easy and then continue a new game on medium difficulty, the kills will carry over and add to your total.