One of the easiest weapon achievements this, thanks to the mid-point of episode 3. This is the first time you pick them up and it's great because you only have flashbangs and they throw loads at you and there are loads to pick up. It's quite possible to get this one before episode 3 ends. Be concise though, don't bother throwing unless you get 3 kills at an absolute minimum. If all else fails there, there are plenty of opportunities throughout the rest of the game to get them: including the helicopter showdown and the rock stage defense.