Well, there's a lot to say about this.
You have used a lot of colors for a work that can be perfectly done with few.
The horns don't seem to have any deph. Instead of shading it's top, shade one side of it.
I'm not really a fan of outlines everywhere. You should shade them and add some AA.
You could also add some texture to the skin of the devil.
He looks like he is falling down. Review it's skeleton and pose. His center of gravity seems missplaced.
His t-shirt is kinda wreid. First of all make HUE variations and don't use so much saturation.
At first try to fix some stuff, post it and then I can continue
cheers
