Another issue, I think, is I can't figure out, do the enemies have too much hp, or do my towers not do enough damage? Either a slight damage increase, or lower the enemies health would be nice.
IMO, if you're gonna have a multiball, why make it so every one that falls takes away a life? It's supposed to HELP you not HURT you, and because of that one small thing: 1/5. It ruined the entire game.