The easy way: At the beginning, after you open the door, simply use your sword to kill the first person you see. The mission will likely fail so just keep restarting the mission and killing the first guard again and again.
Because the target is random, the game will soon select the guard you are continually killing to be the target. The mission will then end and you will have the achievement. It only took me around 5 minutes.
Note: Obviously be sure not to use any of your powers.