I took a closer look...
...best way for the cover would be a fusion between the styles of Richard Corben (muscles anatomy) and Boris Valleyo (colors). But very important... your painter should study the original "Duke Nukem" -Cover. Duke is the important filling in the pic and there is very important acitivity. He shoots some mfucks in the body. This is a very agressive style which perfectly would fit your ambient.



And then.... all balls MUST be greater.
Female characters need much more ass and breasts, wilder hairs, blowing in the wind.

The position of the hero... take his brick in the center as the maximum view point of his movement.

And instinctively right is to use no or not much blood in the pic. Blood or gore would turn down the explosive metal style.

Good work.