|This achievement can be unlocked in just over an hour by replaying a certain level over and over again. First of all load the mission "SALT IN A WOUND", you should have a bow equipped when starting the level with 25 arrows. When the mission starts you have to tear open the door head downstairs in the hanger and then open the next door. Now equip your bow and get ready, as soon as you open the door hide behind the left wall with a good view of the three guards and as soon as one starts walking 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. You now have 3 enemies pinned, you can now go through the level clearing all the guards and once you reach the next door it will say "SAVING IN PROGRESS" this means it will update your number of pins you have done. Simply repeat this process until you have the required amount of pins.|
In order to check how many pins you currently have, go to the menu and click stats, then go down to achievements and find Pincushion. Thanks to Aston158 for the pincushion guide.