This achievement will be easiest if you can get eight friends together on Xbox Live or on the Steam Version. Wait till one person gets the charger, and have one human player stand im front of the chairs ooposite the altar. Then all the other AI and huans stay clear, and charge.
Have One player be a human, and the other four be infected. Have the Human kill the AI characters. When an infected player gets the charger, repeat as above.
(This is how I got mine and its tricky). Have the human player wait until the infected gets the charger. When that person does, have a human move towards the radio a bit until the other AI survivors are out of the way. The infected should spawn out of sight and quickly charge the human through the stairs.
VIDEO FOR TWO PLAYERS: