While I do agree the image should stay as true to the game as possible, a couple liberties could be taken in the graphics-editing department. I'm thinking along the lines of projectiles; parts of the scene could be animated (assuming we're going with the Halo style ad) so that as the camera pans over theme, something will happen e.g. as the clone comes into view it lobs a grenade that lodges into a robot subsequently blowing it so smitherines. How does that sound ?

You could demonstrate digging by having a small tunnel (with a clone actively using a digger at the enterance) that leads bellow some enemy troops, where a robot is firing through the earth.