This game has to be about numbers, not bosses. THe odds of your survival need to be progressively dimished as you go on. Marines need to die so that you have less firepower at your disposal. No bosses. Just a decent in bleakness.
Sure a smart gun at a distance is no match for an alien. But what if there are five of them coming at you at once in the first level and in the second there are ten and so on? How fast can you move and aim to kill them all without getting killed yourself? In that question lies the difficulty and how I think this game should be structured. Very slow moving and full of anxiety with bursts of action that becoime increasingly overwhelming for the player.