Enviromental kills are performed not by weapons, rather by clever use of your sorrounding (e. g. Droping chandelier on someone's head).
An easy method for competing this achievement is to start chapter 3 "Dirty Secrets" (You will need wire cutter tool), after reaching the brothel follow the first blossom you see, she will go to right door and make out with a guard, go straight when they split up and lockpick the door, drop down and follow the vent, you should be now behind the furnace, locate the "mechanism box" and cut the wire, backtrack to the front of the furnace where blossoms are coocking opium. On the right side is the lever, when you pull it the furnace explodes and everyone in the brothel gets high and passes out except you (Garret is expirienced in this kind of stuff
). This gives you 20+ knockouts, wich is more than enough.