To get the shatter kills you need to get the Winter's Howl
from the Mystery box. Once you have the Winter's Howl I would recommend Pack-a-Punching it.
Best way of getting the kills I found was to train the zombies in the War Room
. Ideally for this you will want to have the Monkey Bombs along with the Equip Mint Elixir
. This will allow you to get a max ammo essentially on Monkey Bombs.
To get the kills simply shoot the zombies
with your Winter's Howl/Fury
until they are frozen in place. Then shoot them with your second weapon
, a good weapon to have is the ICR-7 as you can buy ammo for it in the War Room.Call of Duty: Black Ops 4 Breaking The Ice Achievement Guide
I hope this helped you out, thanks again!