Easily done late on in the game. Easiest way I found is once you have the skills "Double Charge" and "Fire Arrow."
Remember the Berserker is the Uruk that will counter all of your regular hits. He usually has two axes . The axes will show up red in his hands when viewed in Wrath Mode.
Once you have those two skills, find a group of enemies that has a Berserker. Get your combo up to where your sword is charged and then use fire arrow by holding
for your bow and then press
to shoot a fire arrow at him, to set him on fire. Then immediately use your finisher (
+
) to kill him before he dies from the fire itself.
Be sure to be standing somewhat close to him to be sure that the finisher connects with the correct enemy.