Here's some critique from me as a comic artist:
This cover doesn't give us a setting. It gives us a protagonist and a vague antagonist, but nothing much else. You wouldn't get to publish this comic, no matter how great the inside work was, with that cover, by any publishing house, I'd say. You're missing a great chance to give us a more pulled out look where you add topography and buildings, as well as a few relevant props to the characters. What is horn dude going to do with that upraised arm? Is he going to strike the vague antagonist? Why not a weapon?
On the technical end, you have some other troubles. First of all, your figures are not obeying the forced perspective. The upturned arm especially is completely broken off the perspective you're using. I could edit when I have a bit of time. Those are the perils involved with drawing in a semi-realistic style. You have to follow through on everything. Then, the guy either has a rib-cage, or he has abs, as far as I'm concerned. Not both. If he has a cavity where his stomach should be and his ribcage has broken through, then he doesn't get to have abs. The rendering on his ab area is supersharp and nothing else on the image is. Another burden with a semi-realistic rendering style is that your image should have a mostly even finish. In this case I would suggest adding more detail to the whole thing (as in, props, geometry, buildings etc) and then rendering everything on that level. Which is a 10 hour endeavour. But them the breaks if you want to *illustrate* comics. There's a reason a lot of us do black and white work!