That is really cool! nice work. I would crop off the empty background & maybe fade in some noise/texture to the foreground directly beneath his feet. So it feels that he is part of the surroundings, wont take 2 mins just in time for your competition.
Exquisite detail on the Marine himself, but I have to note that the lack of detail in the background gives it a very unreal look; there should be more objects and details on the forefront of the terrain.