s, reload, and punch. There's no need to fire your weapon, as your character will fire automatically whenever you have a zombie in your sights.
The missions in Zombie City: Survival last about two minutes. Normally there are three different objectives to do in that time, and you'll earn a star as you complete each one. The objectives can vary greatly: you might have beat the level in a limited amount of time, kill a certain number of zombies, or get a certain number of headshots.
Overall, Zombie City: Survival is a decent third-person shooter, with good graphics, a wide variety of missions, and a vast arsenal of weapons that you can unlock as you progress through the game's story.