[Manny]

Some serious tips:

-strafe around, especially when an enemy is right infront of you. Try to keep your aim on that person even when you're moving around. Don't just stay there and shoot. Only be still if an enemy doesn't recognize you (if you get him from behind).
-reload whenever you have time. Don't shoot away half you clip and keep going without reloading. Go behind a box of some sort and reload.
-play it slowly. Don't be the first...idiot in the group that always gets killed by incoming grenades.