Get 5 kills with Shank in the Kunlun Mountains mission in the Campaign
At the beginning of Mission 5: Kunlun Mountains, you'll be given a shank and then told to shut off power in a tower. Just draw your shank, and sneak around the the outside of this area, trying to stay out of sight from the enemies. Sneak up to enemies, and press melee to kill them. There are 7 enemies in this first area for you to kill, and you can melee them even if they spot you.
Additionally, there are more clueless enemies near the end of this level where you can get more shank kills.
After getting 5 kills with the shank, you'll unlock the achievement/trophy.