Easy Kills As Infected In No Mercy: The Sewer:

The Survivors must jump into a sewer hole at a certain point. As an Infected, use this to your advantage. While playing as a Smoker or Hunter, hide behind one of the large canisters in the room. Once one of the Survivors open the hole and are done checking the vicinity, they will jump in. Due to the fact 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 or her. By doing this, he or she will not be able to be saved, unless someone throws a Molotov cocktail or pipe bomb.