I tried the map suggested in the solution above, but I found that there weren't enough canisters in convenient places on this map, Also, when you did find them, the enemies seemed to be in tough locations to get a good shot on them. So, after several attempts, I decided to switch maps to Highrise...and that was the key!
Start an Arcade game on Highrise: From the spawn make 2 right turns to find an FX (fire-extinguisher) on the wall...grab it and you can toss it up to the second level of the Loot room to nail one to several guards, just try not to kill your teammates! After that run upstairs and out of the room above the Loot room, and youll see another FX. Grab it and use it in the next room on one or more guards. Clear this room and the next and you will come to the left and right doorways that lead outside where many guards are waiting to ambush you. On both left and right walls are FX's! First go to the right and grab FX...run to the top of the stairs and WAIT...give it a few seconds and toss FX about 5 to 10 ft outside the doorway and shoot...this SHOULD net you at least 1-3 kills as there are 3 cops on their way to greet you! If you havent got the cheevo yet, run back down stairs and thru the left staircase grabbing the FX on the way and use it on some cops outside...I think there is one more FX in the middle corridor, between the stairs, before you go outside...If still no cheevo, die and start over...I think whatever kills you got in this round may carry over to the next, but I'm not sure...at any rate, you should get this cheevo in no time using this method!
Good Luck and Happy Cheevo Hunting!