The biggest challenge is to survive for as long as possible. The best way to judge your performance is by getting your name enlisted in the high scores chart. Probably the biggest challenge to master while playing the game is to get accustomed to the controls required for the game play, as they are many in number. The top down view of the game gives a very clear view of the approaching zombies from all directions. You only continue fighting as long as you have health left within you. Move efficiently and shoot accurately. The best way to take down the enemies is by shooting them on their heads.