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 GuideI hope this helped you out, thanks again!