Zombie Army Trilogy

This is very easily obtained in Horde Mode. In solo or with friends load up Dead End with the difficulty set to Cadet and the enemies set to 4. When the mode loads up run forward quickly and to the left to grab the Preacher then make your way back. Because you have it set to 4 player enemies you will have an alleyway full of Zombies. Use the Preacher to help clear out the Zombies that form around you and the Sniper Rifle for the ones at the entrance to the alleyway and you will have this is no time at all.

If you fail to do this at the first attempt there will be plenty more Zombies but limited Preacher ammo but feel free to restart the mode at any point.

While I applaud Lil Miss Cherry for creating a solution capable of being accomplished with friends, the reality is that this achievement is much easier to accomplish in single-player.

Load the level Labyrinth of the Dead, with the difficulty set to Cadet and the Zombie Density set to 4 players. Bring the shotgun with you, as well as a rifle and pistol of your choosing. Explosives are irrelevant.

When the level begins, run down the hallway avoiding zombies until you arrive at the machine floor. Hang a hard right and run until you reach a dead end. At this point more zombies should spawn. When this occurs, run back to where you entered the stacks of machinery and wait for the zombies to reach you, allowing them to cluster into a group. Following this, equip your shotgun and fire away at the group, while backing into the tunnel you began the level in to avoid being hit. (Or simply kite the zombies at your own discretion). By the time you expend all shotgun ammo, you should have at least a 20 kill combo. From here you can use your sniper or pistol, (if the latter I recommend the Webley), and complete the remaining kills.

*My reason for recruit difficulty relies in the compensation for bullet drop. You should be able to pick off the remaining kills at the appropriate distance with a rifle, using the Empty Lung mechanic to appropriately slow time.

Cheers.

* A side note, your combo is only lost if you miss a shot. The aforementioned shot does not need to kill either. Also, for those confused about the path indicated, (though it is quite linear), I have attached the link to one of Zasta360GameTV's solutions using the same route, albeit for a different achievement.

This achievement can be done so easy on the horde map: Dead end. (Easier with friends) just sit at the very back of the alley way, pull your sniper out and shoot at the zombies at the very back of the alley, whilst your friends pick off the ones getter close to you