Edit: According to what I have been told you can also get the achievement by shattering zombies with snowballs which is much easier then getting the Tundra Gun
Original Guide:
For this achievement you'll need a Tundra Gun (The Ice version of the Thunder Gun) there are two ways to get this weapon:
1. Get a free one:
or
2. Get it out of the mystery box
Now just simply build up a horde of 10 or more zombies and freeze them