Zombie Army Trilogy

As found in Sniper Elite Nazi Zombie Army 2, play "Crucible of Evil". When in the Fuhrerbunker, go down to reach a hallway with rooms. You will hear a phone ringing. Enter the first room and use the phone.

Loot the chainsaw zombie.

Part 2, Chapter 5 "Tower of Hellfire" features an unlimited number of enemies spawning in close proximity to each other. Progress through the chapter until you reach the top of the tower and the Occult General. He floats above a platform that spawns the enemies. Use explosives to kill them easily after the Boss fight begins. Ignore the General and only kill the enemy spawns. Run up to the platform between spawns and set landmines, dynamite, or trip mines. Also toss grenades into the group after they spawn. After all of your explosives are expended, including restocking from down the stairs, go back down and use the mounted guns to get more kills. Remain on this gun until you are killed, then repeat the process. You will respawn just before the Occult General with all of your explosives.

The Super Elite is the large zombie with the machinegun. Once killed, his body will disappear fast. There is at least one each level. Run to one when encountered; or load "Tower of Hellfire" and fight until you find a trapped Elite under a tank (just after the long battle with skeletons). Get next to it to kill it; he will not shoot you. Loot the corpse once he is killed.
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In Chapter 3, level 2, "Freight Train of Fear" position yourself in this location when you must survive the assault before taking the train . During the assault many fire zombies will eventually spawn and can be killed with one shot no matter where you aim.