Hitman HD Pack

In Silent Assassin this achievement is gained by using anesthetic on someone. This shouldn't need to much help because you can generally equip anesthetic for any level and sneak (LB) into anyone and take them down.

If you want a great tactic that helps for a lot of future levels I've made a video below using the first level Anathema. The easiest way to grab a guard on the move. Stand in the way of the door, so when the guard opens it the door will hit you and only open halfway (45 degrees). The guard will then clip through the door and not see you. With anesthetic in hand you can easily grab him as he passes and knock him out at the completion of the guard's 'going through a door' animation. If you hold the trigger, you'll automatically use the anesthetic once you're in range. This avoids accidentally using one bottle and letting go of the trigger wasting time and anesthetic.