I know my walktrough are short, so plz read it :D

Boost first your zombies' defense, infection chance and lifespan. When I did this my zombies become incredible and i never loose (maybe one or two times)

For the rest of the game, when you already have 3 infection clicks, a bunch of special zombies and grenades, I just infect two sides of the initial population. Then summon Tank and the gang at the end of the screen. Wait for the police to release your final infection. Use grenades on the remaining survivors when they are out of reach from your zombies.

The game is only hard on the first levels, then on Russia where everyone has a gun. But until you beefed up your zombies, you get a pretty good chance. To get unlockables special characters MJ, Tank, Colonel Sanders, Venom and Santa Claus, here's what you need to do.

Unlockables:
MJ- Destroy the grave in Jamaica (Mexico)
Tank - Kill him in Qandahar (Middle East)
Boomer Santa- Kill Santa in Nuunok (Greenland)
Col.Sanders - Kill him in Kentucky (America)
Ronald McDonald - Kill him in London (Europe)[also found in multiple other places]
Venom - Kill Spiderman in New York (America)

Have fun!! :D :D
Resource :
http://www.dopiephilippines.com/2010/02/infectonator-world-dominator.html (I make my walktrough with some little editting)