I really like the background and colors used here. The shading and shadows were pretty awesome. On the topic of shadows I feel like the shadows of the rocks and his body maybe should have been facing straight back instead of to the back right corner. I understand that there seems to be a strong light source taking it that direction, but the shadows seem to completely ignore any effect that the lightning around Jack Wernicke would have on the direction of the shadows. Also, it would have been really cool to see a shadow of the boss with a darker outline to really give a good feel for how intense the battle is about to be.
I feel like some of the proportions are off a little. The lower body is very long while the upper body seems to be a little short. The arms bug me a little because the forearms are the same size or bigger than the upper arm. The ankles seem extremely tiny for the size of his legs and the neck seems odd for some reason. I think it may be how much of a point it comes to on the top. On the hands the fingers are odd sizes. It's weird that some fingers that should be smaller than the thumb in girth are actually thicker.
Overall it's a really awesome piece and a new approach on a Madness character. It would be pretty cool to see a flash series created using characters in this style.