Mephils takes large amounts of damage, but refuses to die.
Confession here:
https://dl.dropboxusercontent.com/u/24522008/Terraria/MephilsHack.png
And this; I’m pretty sure that the weapon used by Mephils isn’t in the game:
https://dl.dropboxusercontent.com/u/24522008/Terraria/MerphilsHack2.png