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?

Probably the easiest method is the Line Launcher Takedown.

After disorienting an unaware enemy from a distance (6, right-hold, left-click), fire the line launcher (8, right-hold, left-click) at them, then right-click on approach.

A Special Combo Instant Takedown (E) may also be possible, but you need a group large enough to build combo but with few enough guns that you don't just get shot. There will be a small window to drop the pellet and target the correct enemy in-between combo moves.