Very cool, when working with a swarming enemy game like this I would tend to agree with what the previous comment said--you want to keep enemy health to a minimum and responsiveness to a max. Turn four hits to kill to one, and play a little sound and have a little particle effect when the player gets a kill. Yeah, put a machine gun in. You will have a better time balancing that, as in, give the player a sense of power to fight with and then scale enemy difficulty with later waves. Currently I feel as though my slow, single-shot weapon is ineffective and spent most of the game running and not bothering to shoot.
Anyhow, good job and good luck for the future!