This is a plot progression trophy, earned by defeating the Mouth of Sauron in the Journey's End level (last level of the game).
You will need to swap back and forth between your characters; while the Mouth of Sauron is attacking one, switch to the other via Triangle to mount the attack. When the Mouth of Sauron switches targets, you need to switch attackers.
If you try to attack the Mouth of Sauron as its current target, he will simply deflect your attack for no damage, hence the need to keep switching.