This achievement is slightly non-obvious. Once in the second mission, make your way to the Black Market. You need to purchase an upgrade that allows you to create basic bolts out of everyday objects. It is on the second page of the upgrade purchase list, in the second row. Once you have that you can find a fountain pen in the apartment building directly above the black market, next to the bed. To make things simpler and guarantee you're using the pen, save your game, fire all of your bolts, then grab the fountain pen so that it is the only bolt you have (it's not clear if this step is necessary, but it was the only way I got it to unlock and guarantees your only shot is the pen). Head back down and find a guard without a helmet (there's likely one across the street and to the right of the front entrance with his back to you if you haven't eliminated him already). Go up to him (if you're too far away it probably won't work) and shoot him in the head. If you kill him with the shot, the achievement should unlock.