Unless you're extremely proficient with the Bow, this achievement will be quite time consuming, tedious and ever so slightly annoying.
To earn this achievement, you must pin 50 enemies to a wall/object using your Bow. Easier said than done, as the arrows don't always pin your enemies.
The easiest (but certainly not the quickest) way to earn this achievement is to load the mission "SALT IN A WOUND". When the mission starts you have to tear open the door, head downstairs in the hangar and then tear open the next door. Get your bow ready; as soon as you open the door hide behind the left wall so you can just see the three guards. As soon as one starts walking in your direction, pull your arrow back, count to 3, and pin him to the far wall. As soon as you have pinned him pull another arrow and look around the left corner, then before the guard moves pin him to the box and straight away pin the other guard out to the far right to the box. The annoying part is that the second and third guards don't always follow the same route, so you may get anywhere between 1 and 3 pins in this section.
Once you've dealt with those 3 guards, pause and save the game. Then reload the level/checkpoint and carry out the same process over and over again until you have your 50 pins.