just a few suggestions for you
firstly - the rain. You should use a few more different frames for the rain, say at least 3 or 4. This would give you a more realistic looking rain, as a few or your rain drops were simply piveting in place.
secondly - On the fade out of the heads, you've faded the hair layer to fast and you can see top of the guy's head through his hair.
thirdly - the hair and power up movements. As with the rain, you should of used a few more frames then what you did.
I hope this helps you out for the actual series =)
keep up the good work!
p.s. great choice of music, dead bodies everywhere, top choice!