Too easy. I just kept my money and bought the M4 and just throw mines for the biggest zombies. Also just timing your jumps and shooting those birds out of the sky fast makes this a cakewalk. Don't stay on top of your car and don't buy car health upgrades. You don't need it at all because the zombies will focus on you. So if you stand on your car they will damage it, but if you're just running around dodging their slow attacks with jumps, you never die and your car doesn't get damaged. Hell, the zombie hits doesn't even hurt much anyway. Easy to level up if you take the EXP upgrade and that will also provide you with some health boosts. DON'T BUY THE HEALTH KITS! It's way more effective to take the auto heal upgrade, which ofcourse is easily gettable when you upgrade your EXP first. I completed the game on wave 18. ( I miss clicked and bought the AK-47 first.. XD ) I probably could do it even faster, but still didn't even die once or my car.
People having problems to complete this game are just utter noobs.