The Super Elite the large zombie with the machinegun. They are found in each of the campaigns. There are also some in Horde mode. Consider playing "Library of Evil", "Tower of Hellfire", "The Keep", and "Freight Train of Fear" to do this eaiser. In "Library of Evil", "The Keep" and "Freight Train of Fear", you can find at least four of them. "Tower of Hellfire" has also some of them, including one trapped under a tank (after the fight with the skeletons) that cannot shoot you, that is an easy kill.