The Executioner

It seems slow at first, but that's just a ruse to set you up for its mad dashes and long jump attacks. Using its ax, it'll march toward you to do its dungeon-inhabiting job. It's not so much that it's hard to defeat--it's that the construction of its chamber makes it hard to maneuver around. The Executioner often plants himself on a platform right above you, swinging his axe wildly, leaving no way to escape. Try not to get cornered and you might have a fighting chance.