You will obviously need the Coldfire Aura. I would have at least have Duration, Area 3, and Recharge 3 upgraded. Fire & Ice is optional. It is useful because it freezes the enemies in place, it is bad because the enemies are extremely fragile and outside damage will shatter them. Those shattered will not count towards the achievement. The enemies must die from the damage taken from the aura itself so don't shoot them. The fastest way to get the achievement is to find the armored trucks that show up on the mini-map and damage it enough that the enemies pop out. The majority will come out the back. Go there and stand in between them and use the aura. You may have to use it a second time to finish them off. You get about 4 to 6 enemies with each truck.