Call of Duty: Black Ops 4

Playing on casual will still allow you to get this achievement and all other zombie achievements besides quest (Easter Egg) ones.

Get the Winter's Howl wonder weapon from the mystery box. Shoot zombies and when they turn to ice but make sure you switch to another weapon and shoot them to shatter them. You can also use your knife or grenade equipment but if you take too long to "shatter" them, they will just blow up by themselves. If this happens, those kills won't count towards this achievement.
A pack-a-punched Winter's Howl will help with this achievement as well. Personally, I used a fully upgraded shotgun (repack-a-punched 4 times) to shatter the zombies and to deal with dogs.

You can also unlock this Achievement in "Rush-Mode". You don't have to pay any Money for the box, you just have to wait some time to reactivate the box. get the Weapon and kill your enemies.

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!

There are multiple ways to do this, but I believe this to be the most efficient and easiest. The first and most important thing you want to do is buy the door for 500 going into the elevator hallway and make sure the mystery box is in the main offices. If it is not, simply reload the game until it is. If you do not pull the Winter's Howl before the box gets relocated I would simply restart. You don’t have to, but I think it beats going into the later rounds and hunting the new box down.

Make sure you open everything on the floor you spawn on. It will allow you more room to move around and train zombies.

Once you get the Winter's Howl group and train zombies up so you get the most out of your shots. I knifed or shot the zombies because when I ran through them after freezing them I could not get my achievement to pop. So be safe and freeze them and switch to your secondary and shoot them to break them.

If you are having trouble staying upstairs like I did you can go somewhere else on the map.

This can be done on casual I wouldn’t use bots because you need as many zombie opportunities as possible.