This achievement is pretty simple. Your first opportunity to do this quickly is right in the beginning of level 3 of Contracts: The Bjarkhov Bomb. Take a disguise, a crate, and enter the kitchen. There's a bottle of laxative on a counter by a pot of stew. After grabbing the laxative place it in the stew and then wait. If you want to wait behind the door in the bathroom in order to take out your target when he goes to relieve himself that would be the best idea; while completing the mission. See the video, which is how I began the level.