My strat: Box up the bottom area with 6 defenders and have 3 guys scrounge ammo, health and debris as needed. Upgraded scroungers 1st to about lvl4. Then upgrade speed and ammo of your 6 defenders steadily, while upgrading the scrounging guys as well. When defenders have speed and damage to 10 you start upgrading 2 scroungers' attack and speed as well. Leave 1 scrounger with lvl10 scrounge at ammo and watch your little guys own zombies!!!
 "Zombie Knight"){kind=icon}
 "Upgrade Complete 2"){kind=icon}
 "Orbital Onslaught"){kind=icon}
 "Luncheon Of The Dead"){kind=icon}
 "Book of Mages: The Dark Times"){kind=icon}