The best way I found to get this was to go to
Outlands Canyon and head into the middle area (I'll include a video to show you better).
There you will find many
Tork enemies as well as
Cryo Vines.
http://borderlands.wikia.com/wiki/Tork_(species)
http://borderlands.wikia.com/wiki/Cryo_VineSimply round up a bunch of Torks or other enemies, minimum 3 but its good to have a few extra to make sure, and then bring them over to the Cryo Vines. When the enemy gets close to them it will automatically discharge its frost area of effect.
Once you confirm 3 enemies are frozen, jump into the area and ground slam.
You will get your trophy shortly after provided 3 enemies die due to your slam attack.
If you don't get the trophy, just round up some more enemies and try again. It took me 2 attempts to get this.