This is a guide that will help you getting the achievement: I ran it under a cold tap, They're all messed up and I have given them the last Reichs. Because they're all the same thing to achieve but just with another number on them, i'll be doing just 1 guide for this. The thing that you have to do to achieve this achievement, is to kill 10, 15 and 20 zombies with 1 grenade in a 4-player co-op mission. If you want this achievement just follow these steps:

[NOTE: You have to be with 4 people in a group to get this achievement so make sure that you are in a full team!]

- First, play the second level: Cathedral of Resurrection.
- Now play the game how you would normaly do until the first checkpoint.
- If you get out of the checkpoint in the house, you will have to leave the house (throught the side of the house). You will have to go 2x left, and directly after the corner of the house (the same house as the house from the checkpoint) there will be a fence at your left handside with around 20 zombies behind it.
- Make sure that you will the first arriving there because others might shoot some already or open the fence!
- If you get there throw your grenade right in front of the fence. (When I tried to throw it behind the fence it bounced back, so...)
- You should be able to get 20, but you will probably get a little less... but that's still 2 achievements at once! (Because it's hard to get all the zombies for you alone with 3 others around you, and throwing the grenade might be hard because for killing 20 zombies it needs to be on that right spot).

Good luck with getting this achievement!