The Sandman achievement is unlocked by successfully subduing a person without killing them. Go up to any unsuspecting guard with your weapons holstered and subdue him. Just be sure not to break his neck. This must be done on a level aside from the first mission.
You can do this on the King of Chinatown. Walk through the large doors into the square and take your first left. There will be a guard behind the boxes on the right that you can't see. Wait for him to go up the stairs to the right and stealthily follow him up. When you are on even ground, subdue him. If you are on the stairs, you will instead start a fight with him or instantly kill him.
