Great art and coloring
I like the designs of these characters, and the coloring and shading is fantastic! However, you did ask for constructive criticism, so here we go:
-I'd suggest you add a few more of the little demon things at the bottom, to make it clearer that the purple monster they're running away from is really big and on the rampage!
-Is the purple guy supposed to be floating? 'Cause that's what it looks like- like he's just hovering above the ground.
-The other demon on the bottom right looks out of place, and doesn't appear to be involved in the scene shown on the rest of the image. Either nix him or integrate him better.