Each of the enemies is said to be intuitive A.I. able and willing to use cover, set up ambushes, and use the environment to get every vantage point against you through the game. You'll see enemies ranging from your standard Grunts to the specialized weapons forces (Sniper, Heavy Rockets, etc.), to phase (ghost) enemies.
Many of the art showcased in the gallery below are beautiful concept drawings of the enemies - but there are also fully rendered screenshots from in game so you can see what these baddies will look like when you're face-to-face. All of the gallery images show one thing for sure, these guys aren't fooling around.