Monkey See, Monkey Doom 15
In Diner Turned, retrieve a chained Cymbal Monkey in a Public Match.
After killing a bunch of zombies consecutavly, a cymbal monkey will randomly drop. You need to pick it up and throw it at a small group of zombies (Around 3 zombies). It will chain them together and kill them. Once they die, another white monkey cymbal monkey (white glowing) will spawn, pick it up and the achievement will pop. Thanks to SIMPLYMIDGET for the info regarding this trophy.
To add to this achievement, 2 other people had left the game and i found myself killing the same guy over and over and ended up getting the cymbal monkey. I threw it and it killed him and then the other cymbal monkey appeared and i got the achievement so you definitely do not need more that 1 zombie to do this.
Also the best way to get the Polyarmory is through luck and getting insta-kill as this will make it so easy to obtain the achievement. First time i got and i unlocked the achievement with no problems.