I was watching some gameplay clips of AvP2 (game), where the aliens are in massive numbers, but go down fairly easily to gunfire, such that both the Pred and human characters obliterate them at a rate rivaling Serious Sam at some points.
They never "end" and the plot is very clear that they are overrunning the place and winning, but they seem to be seeing if they can run the humans out of ammo.
Even when you play as the Alien, while you do quite a bit and are very powerful, you have your own expendable xenos around, and a couple of the objectives even involve you using your fellow xenos to soak up sentry gun fire.
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This goes back to the original "Alien" versus "Aliens" comparison. One bug, versus hundreds. The xeno on the Nostromo is the perfect predatory organism with no weaknesses. The ones on LV-426 keep coming after the humans despite anything the humans, with heavy weapons, can think of to do to hold them off, but they take massive losses and go down easily to gunfire.
Which concept of the creature do you think is the better route?