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.
Melee attacks
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.
Nightmare mode
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.
Power-ups
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.
Ranks
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.
Combos...