Kill means = knock down + thirsting (box)
There're 2 possibility:
1. Let the gas kill them (knock down + box)
or
2. While they're inside the gas, kill with the gun. (you seems doing this one)
What is the exact verbiage?
Most likely #1. You have to use gas to knock and until they die.
If its just to kill a enemy under the effect of your gas its quite straight forward. You want to be in an enclosed area with a lot of gas traps set up. (Dont forget you can trigger them just by shooting to blanket an area) since the gas doesnt effect you, you'll have a huge situational advantage.
Could try the good ol' caustic bunker drop and see where that gets you. 😛
