How do you do a takedown after a smoke pellet?

One of the challenges in Batman: Arkham City is

Use the Smoke Pellet to disorientate an armed enemy and follow up with a Takedown

But how exactly do you do this? Every guide I’ve found just says something along the lines of find an armed enemy, throw a smoke pellet at them, then do a takedown. But there is never any takedown option. I’ve tried just punching them, standing in front of them, standing behind them, crouching, and there’s never a takedown prompt. They just stand there flailing about a little until the smoke clears.

So what exactly are the inputs required to complete this challenge?