Upgrade the Coldfire Aura to freeze and burn your enemies at the same time before going for this trophy. Using the aura when this has been achieved will freeze the nearby enemies, drain their health slowly, and possibly shatter them eventually. You will see a counter appear when you reach the quarter-marks of the trophy itself. This way you don't have to worry about your targets attacking back. Be careful though, as it doesn't stop other enemies from destroying the enemies around you.
To have the kill count, they have to shatter on their own.