Mousetrap Fuse Achievement
Use a trip mine to kill an enemy who is trying to assault your positionYou will probably get this naturally if you are using explosives during your playthrough, but you can get this easily by playing the 'Prologue'.
During the 'Prologue', if you have the tutorials turned on, the game will tell you where to set your first trip wire. After you have eliminated the Major-General, enemies will be on high alert. Chances are that an enemy off screen will run into your trip wire and die, unlocking the achievement.
If for some reason it doesn't, you should leave a trip wire in a doorway or in an area that an enemy might run through and when they trip it, it will explode and the achievement will unlock, even if it is off screen.