I reccomend doing this towards the end of the game so that you have the majority of abilities available to you, for this method you will need:
Fire Arrow (Wraith Ability)
Double Charge (Ranger Ability)
First find a Berserker, they have two weapons as oppose to one so should easily be distinguished.
Next, get your combo in the red so that you can fire a flaming arrow.
Make sure the Berserker is right next to you when you shoot otherwise you won't be fast enough.
As soon as you shoot the Berserker, press
+
at the same time to execute him. This must be done very quickly otherwise it will not register as a kill, so just spam the execution.