Aim for headshots for best results. You will get a cash bonus and kill zombies easily. To get a higher head shot rating, fire your machine gun then slide the gun straight to the side. You will be able to hit their heads for easy headshots. However, this will require more ammunition than aiming directly at the head. You can also aim at the chest area, especially at long distances when aiming at the head proves difficult.
Gold
Gold is the game's premium currency. Tap the "Free Gold" button at the top of the map screen. Complete offers and watch advertisement videos to earn gold. The videos refresh every ten minutes and are not limited, allowing you to earn as much gold as desired.
Play in the casino to get free cash and gold. Completing quests and earn achievements to earn gold.