Left 4 Dead

I setup a local split screen match. Have the second controller be a survivor and the first controller (which your assigned in to live with) as infected.

I did this on No Mercy on easy.

Have the survivor shoot 2 of the AI players. Go down the stairs in the first level and around the corner. After you've cleared the room of a few zombies, walk forward a bit such that the AI player is positioned away from where you'll spawn in with your smoker. You don't want the AI player to see you right away and shoot you before you have a chance to attack.

Now, using the first controller(infected), spawn as a smoker and then constrict the AI player. Note, as this happens, a horde of zombies will come in so you may need to switch to the second controller(survivor) and kill a few of the infected. Be careful not to shoot your smoker.

After the AI character is dead, use the first controller(infected) to constrict the survivor using the second controller, who will, obviously, not fight back.

Took me about 10 minutes to do.

To boost this achievement, do a split screen session with the profile needing the achievement set as the Infected and the other as any of the survivors. Start the game on easy and clear out the immediate area of zombies. Then reset the difficulty to Hard and proceed to have your survivor eliminate two others. This leaves the controlled survivor and the AI. Change it back to easy and get over to the Infected controller.

As the infected, spawn and intentionally get killed by the lone AI survivor if you spawn as anything other than the smoker. Once you spawn as a smoker, try and get the drop on the AI survivor and snag them. Since the controlled survivor is the only one left, there will be no fear of getting shot while doing this. Once you drop the AI survivor, go in for the hapless controlled survivor.