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.
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.
Kill green backpack zombies first. Their backpacks hold power-ups, such as health, double damage, and others. You can also spend gold to purchase temporary power-ups that last for only one level. They work just like the power-ups from the backpack zombies, except they will be activated when you start.
To get a three star rank on a level, complete the following goals.
1. Complete the level.
2. Collect all five intels on the level. They are on the tall zombies with glowing backpacks.
3. Complete the level with full health.
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