Once you spawn as a Charger wait until you have a clear shot at the survivors. Press the to charge and drag off a survivor and once you stop, the Charger will automatically start slamming the survivor into the ground. If you have managed to get far enough away from the other survivors the animation will keep going until the survivor is dead.
System Link: Versus
Kill off two of the survivors in the saferoom, exit the saferoom once the second controller is the Charger. Charge at the AI survivor and watch as your other team mate's partner is pummeled into the ground.
You need to be playing as the Charger to earn the achievement. Use a guest/second controller to play as the survivors.
Alternatively you can use multiple controllers with several players to build the team numbers. If you are playing with two people you need to have at least one controller set to the Infected team. Doing this allows you to pick between the Infected you need achievements for. Make sure whoever needs whichever achievement players as the Infected.