For each zombie killed within a certain period of time, you will build a combo multiplier. At a certain combo level you will begin to earn double coins and double points for each kill. Continue building it to eventually earn a triple multiplier. Keep moving to constantly attack zombies. So as long as your bullets are hit a zombie, your combo will not reset.
Your knife can be used to melee weaker zombies, killing them instantly. It starts with a 9 second cooldown, but can be upgraded for faster use and increased strength.
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After completing a level with a three star rank, you can play the same level again in Nightmare mode. It features twice the number of zombies with double the power. Completing a level in Nightmare mode will award you with diamonds, the game's premium currency.
To get a three star rank on a level, complete the following goals.