This is most easily done when you're down to one, fairly weak opponent (e.g., if you come across bandits, kill all but one). Simply face the one left and hit the
every time his health bar flashes to signal an attack.
You can also do this in the tutorial early in the game where you do the training session with Vesemir. During your combat with him, he'll have you break down a shield and then ask you to hit him with one more Aard. Instead of hitting him with the final spell, just wait and he'll continue to do simple attacks that you can counterattack.