For the final level on the mission that you have to use less than 80 shots: You have to use the grenade launcher. Go back to level 7 and buy it if you don't have it and buy ammo throughout level 7-10. It will take you about 30-40 grenade shots to kill the final boss. Good luck!
It's easy getting the badges without money. It'll take you a month or so. You just have to be active. I just need the last badge and I've been playing for about 4 weeks.
 "Zombotron"){kind=icon}
 "Red Remover Player Pack 2"){kind=icon}
 "The Final Death Wish"){kind=icon}
 "Cursed Treasure: Don't Touch My Gems!"){kind=icon}
 "Generic Defense Game"){kind=icon}
 "BUMs: Back to School"){kind=icon}
 "Viricide"){kind=icon}
 "Bloons Tower Defense 4"){kind=icon}
 "Homerun In Berzerk Land: Berzerk Ball"){kind=icon}
 "Heroes of Gaia"){kind=icon}
 "Dream World"){kind=icon}
 "Kongai"){kind=icon}
 "Continuity"){kind=icon}
 "Pandemic 2"){kind=icon}
 "Balloon in a Wasteland"){kind=icon}
 "4 Differences"){kind=icon}
 "The Easiest Maze Game Ever"){kind=icon}