For me the problem with the composition is the huge structure in the centre of the image, its darker than the other buildings in the scene and being texturally rather featureless it creates a void in the image, I'd be inclined to move it further back in the scene and dirty up its texture especially at lower levels and perhaps obscure it slightly with a smoke cloud.The other void in the picture as I see it is the space just behind the tank, true the Necrons are firing into it, but there's nothing to suggest that there are any Ultramarines firing back from that space, perhaps a helmet just visible above the tracks.

Not sure about Necron "blood" if they ooze at all, I'd always thought it'd be a similar colour to the glow on their ribs