Concept:
http://dougbot.com/forum/meatHoleDemon.jpg
A real wizard, physically he's taller yet thinner than a Shambler.
A number of magical ranged attacks as well as summoning abilities and physical regeneration.
When the player first sees him he's seemingly dead, impaled on a lance jammed into the wall, on their approach he disimpales himself and uses his regenerative magic to recover from the wound.
Throughout the battle the player wears him down despite his regeneration until he resorts to destroying the floor, levitating himself around the arena. Heavy attacks can disrupt his concentration though, causing him to fall into the void revealed below.