There is two ways to achieve this, you can use a Northern Wind bomb or the Piercing Cold, which gives you a 25% chance to freeze opponents when Aard is cast. Using the bomb is the easier method as it is guaranteed to freeze them, whereas using the sign is not.

Beat down the enemy until they are low on health and then throw a bomb at them or try and freeze them with the Aard sign (there is a chance they will die doing this). Once they are frozen finish them off with a crossbow.