Fortunately for you, this trophy doesn't refer to just the "ghost" enemy in the labyrinth, but all ghostly enemies in general, so the skeletons count too. You should get this through natural progression.
If you have not played the end game area of the base game you won't recognize them but the skeletons are a re-skin of the citadel soldiers, and therefore pack massive damage for faith and agility builds. Watch out!