I'm trying to find pieces of coal in the goblin caves and I came across a lock screen with four ghosts. They pretty much kill each other but then there is one left or sometimes two left. How am I supposed to kill them when they are invisible most of the time and they can kill me with 2 or 3 hits before I can even see them? All I have is the lantern which doesn't seem to do much damage. :/ Is there something I'm missing or some kind of strategy to defeat them?