In level 1 of the No Mercy chapter, the Survivors must jump into a sewer hole to enter them. As an Infected, use this to your advantage. As a Smoker or a Hunter, hide behind one of the large canisters in the room. Once the players open the hole and are done checking the vicinity, they will jump in. Since they must jump in one at a time or get stuck, make sure you have a clear view of the last one in line. Once all three are down and the fourth Survivor is still up, pounce or grab him. By doing this he will not be able to be saved, unless someone throws a Molotov or a pipe bomb.