At about 300 zombie kills, larger enemies will begin to appear. Kill them immediately, and do not wait the proximity warning. Use the 105mm or multiple 40mm shots.
Civilians
Each civilian that successfully makes it to the bunker will result in coins. However they are best left ignored. When you see zombies attacking a civilian, do not intervene to save time.
Recommended weapons
To help civilians survive with accuracy, use the 25mm. Using a larger weapon near a civilian will increase the chance of accidental death.
The 105mm is best used to take out large concentrated groups of zombies. Fire the 105mm while waiting for the 40mm to reload.
Use the 40mm most of the time.
Upgrades
Enter the "Extra Upgrades" menu to access the Zombie Bounty upgrade immediately. Spend coins there to increase the amount of money earned per zombie kill.
Guns can be upgraded in four ways: damage radius, reload speed, rate of fire, and bullet velocity. Be careful with the damage radius upgrade, as it may allow civilians to be unintentionally killed.