Your objective is to kill all 5 bosses without dying. When you die you’ll be taken to a store screen where you can use money you earn from killing zombies on a number of upgrades and weapons. You get as many attempts as you need, and the game will tell you how well you did based on how many times you died before finishing the final boss.
As Always, Good Luck and Have Fun.
Aim & Shoot: Mouse
Move: WASD or Arrow Keys