This was very nicely done! Having to leave one specimen alive definitely adds a neat wrinkle to the formula, and constantly changing up your arsenal is a great way to keep the game interesting.
My only complaint is that the rifles (the ricocheting ones) were more of a hindrance than a benefit because of their tendency to bounce and hit targets you were trying to keep alive. Trying to keep track of what moving targets they'll hit after the first ricochet is very tough.