Video Description:

Thief - Health Hazard - Kill or knockout 10 people using the environment [Silver / 25G]

The best place to do this is in chapter 3. After entering the brothel, go down the hallway on the right. There's a big machine in the middle of the room. That's the opium machine. It will knock out everyone in the entire brothel and immediately grant you the trophy or achievement. To do this you need to manipulate the machine with the wirecutter. In the same room is a locked door. Pick the lock and you'll come to a room that leads to the area behind the opium machine. There's a panel on the wall. Open the panel and cut the wires. Now return to the room with the machine and pull the lever to knock everyone out.

You can buy the wirecutter from any merchant in the City. Merchants are marked with a white bow & arrow symbol on your world map. You can replay this chapter at any time.