7. Sanctum Knights - Dark Souls II
In a game like Dark Souls II, the last thing you ever want to come across is an enemy you can't kill. Far from a series that encourages running away at the first sign of trouble, you'll probably assume there to be a trick to this, but until you figure it out, you're going to have quite a bad time.
The game's first major DLC, Crown of the Sunken King, introduces the Sanctum Knights, which, at the time you first encounter them, are ghosts. And, as we well know from Dark Souls 1, ghosts cannot be harmed.
It's possible to make these ghosts tangible again by finding their armour and releasing their essence from inside... but let's assume you didn't work that out straight away. Is all hope lost? Like hell it is!
In a large room containing many of these Sanctum Knight ghosts are two bottomless pits side by side, which you can trick these annoying apparitions into by baiting out an attack from them, just as they step over a corner.
If you have the patience, you can even clear the whole room this way.