upgrade lifetime,speed and infection chance mostly and upgrade hitpoints some and you should kill easy. a fast zombie can infect like 5 people in 2 seconds.
it was probly the funnyest game i ever palyed but its so sericous and the winning will get you non stop raoring to listen to just wat i wanted and its a 5/5 at the start guy falls onto the girl. awsome game and still funny to the end and they didnt make it dry by the end of the game