This one is pretty simple. All you need to do is either craft the Northern Wind bomb to use on an enemy or use Aard with the upgrade that allows for 25% chance to freeze. To make things easier, go with the Northen Wind bomb as it is guaranteed to freeze enemies.
Find and common enemy and get it down to a sliver of health. Mind that you don't burn, poison, etc. runes on your weapon as it could kill the enemy before you can get the crossbow kill. Freeze them with that last bit of health and hit them with the crossbow arrow. You will want to do it in one shot but if timed right you might pull off two before they unfreeze. Once they die, the trophy will pop!