You need atleast 3 or more walls for this to work, and a crowd of enemies of course, then you charge the frost ability on the walls and that should do it.
If that doesn't work then, I'm not sure what went wrong but thats how I got it, dunno if maps affect the achievement but i got it on the Glass Houses map while messing around with the Hammer and the Freeze RAY
![](https://i.ytimg.com/vi/__-jre4Y5ys/mqdefault.jpg)