NOX: Kill 1 enemy as Caustic while they are gassed not complete

Sorry to necro this post, but these challenges are still frustrating to me as a Caustic main. All other uses of the word "kill" for the purpose of challenges, seems to refer to the moment you finish their standing hitpoints, to knock them. However, with this specific challenge it seems to mean the moment you finish their knocked hp, to turn them into a deathbox. I'm not sure why the inconsistency. If someone demonstrates different, I'll rescind, but this seems to be how it works, to me.