The Kraken is a weapon that can only be obtained from the mystery box to my knowledge. To get this achievement, you must kill 9 zombies with one shot from the Kraken.

Getting the Kraken to drop from the mystery box is probably the hardest part of this achievement, but once you do get it, it's not that hard to kill 9 zombies with one shot. I recommend attempting this achievement on casual solo. You can use bots if you want but I found it to be easier without bots because they tend to kill the zombies that you are trying to herd into a group.

The Boiler/Turbine room, the cargo hold, or the dining hall are all great places to chain up a group of 9 or more zombies due to how compact the areas are, especially on later rounds where hordes of zombies are chasing you around. Once you get a big enough group of zombies chasing you, then let them have it.

Worth 15 Gamerscore.