this only works for the first three levels, after this its basically setting up turrets, placing as many roadblocks as possible, and spamming your units as well as spamming your 1 key.
Here's my strat, when you started immeadietly upgrade the Generator while move to the right and catch up with the zombies and soon as you see them, shoot them.
keep upgrading the generator until you see the spider woman zombie, once you see her go back and shoot her couple times while you go back.
Once you got to the frontline of your zone place the big turret as well as keep upgrading the generator, and place more turret and so on and so on.
Once you got at least 5-6 big turret next to each other, place the barricade up front of those turrets, You just need 1 barricade up front of those turrets and 1-2 barricade inside the turrets or withtin the turrets and also keep upgrading the generator!
if the barricade broke, build it again.
once you got enough money, i reccomend you to upgrade machine gun lady, after quite some time your generator at least already max level.
After that, farm more money, once you got enough at least 30000, buy the penguin, 3 machine gun lady, and 2 medics.
That's all.