Here is a video of how to get the trophy made by myself, but before a little explanation on what to do.
- Horde Mode
- Dead End Map
- Cadet difficulty with 4 player enemy settings.
- Only put 4 hand grenades (The rest doesn't matter)
As soon as you get on the map, move forward until you get to a little corner where the main ammo refill is located. On a crate is the almighty Preacher with 2 restock of ammo. With this, and 4 grenades, and in the very spot you pick it, bring mayhem across the two sections the zombies will come. For maximum efficiency on using the preacher, crouch before you shoot.
Once you have run out of preacher ammo and grenades, restart the wave and repeat.
PD. This also works for the 5000 Kills trophy.
Pd: Please, if you vote down say why so I can improve the solution. Thanks a lot.