Shoot the zombies by clicking on them quickly before they take away your lives. Normal zombies take away -1 health, Chargers take away -2 health and Tanks take away -4 health. When killed, Tanks give you 2 extra lives, and Chargers and Normal zombies give you 1 extra life. Be careful: zombies may hide behind the gun, the edges, or behind the Lives variable. The Chargers are also hard to see in the background, so make sure you look closely for them when the 'Charger zombie is attacking' sign appears.
Credits to Left 4 Dead for Charger zombies and Tank zombies.
