For this one, you just simply have to use the Monkey Bomb once. You will get it from the Mystery Box. Whenever get the Monkey Bomb from the Mystery Box you will have three Monkey Bombs. You just have to use one. To use a Monkey Bomb, you must press , you can hold it down, and you won't throw it. It's just like throwing any other special grenade, except, you know, it's a monkey.
Once you throw it, it will start making a lot of noise. It lures the zombies towards it so if you are surrounded it would be ideal to throw it then. After a few seconds, it will blow up, killing any zombies around it, remember sometimes it won't kill all the zombies surrounding it, so just be prepared.