Not bad...
I really liked it- non-the less, I'll explain the score.
First I'll go over the things I did like:
I liked the storyline, as vague as it was, it was pretty cool. For once, it was a fair amount of original animation. It has the same concept as some other movie/game titles, but it varies with the creatures and character design. The sound effects were pretty spot on and the voice acting wasn't bad either, usually screams sound like a little kid yelling slightly while fading into the background, but in your case, it actually sounded more "realistic", it reminded me oddly of Left for Dead though.... Since you seem kind of new still, especially with an entire flash in a non-"sprite"-like animation, I think you sparred pretty well.
Though, the things I didn't like, but found amusing, would be some of the cartoon panning of other objects from different sources... such as the sniper rifle from halo and the rocket launcher from Call of Duty: Modern Warfare. The creatures were... "unique" to say the least, they were pretty well drawn, considering the rest of your animation. But parts of it that I didn't like was the way it's head would pan back and forth and nothing but a little stretching of the face, so to speak, would occur when their necks would move back and forth. Last thing that kinda bugged me was the opening song, Clubbed to Death. It kinda fit, but in the back of my head it kept pounding "No.". In this type of scenario I suppose it's okay, but I, personally, would prefer something different, more grungy, something that would make you feel your in a camp of survivors or something, not the Matrix or Blade.
A few things that I didn't like at all would be the arms and torso connection of the troops with the AKs. It looked too awkward to me, they seemed to be too far out, like the shoulders were too large for the rest of the body, making more of a "T" shaped body. Their mechanical movements seemed odd also, they also seemed to all breath in unison, I personally don't like that because it just makes it look they're all just copies of each other, rather than unique characters (even though they all look the same..) Idk, there seemed to be a lack of a lot of detail in the background, i did see better detail toward the end, but all around there was a lack of it. I'm not asking for shadow's of the characters or anything you already kinda had that, but just some of the overall background slides... Other than the ally, there wasn't any shading through the city or anything, real lack of detail IMO.
Overall, this animation kept me entertained and that's the main part, I just ask for a little more detail and a little cleaner animation tweening. Maybe when your done with the second one, go through a little more and add some detailed shadowing or something, the more you clean up your animation the better you'll get at animation. This isn't horrible, so don't get me wrong when I ask for a little improvement, I try to pick apart all that I can so I can try to help you improve. Not quite sure I did so well this time around, but I hope it helps a little or something.
Animation: 3/5
Graphics/Art: 2/5
Music: 2/5
Originality: 2/5
Other (I.E. voice acting): 4/5
Overall: 6/10
Keep up the work, you have some potential, you just have to force yourself to be more artistic when it comes to detailing things, that's your biggest problem i think, that and maybe tweening.