The objective is simple: Kill all the baddies, Zombies through to the Devil. You need to unlock new battlefields, characters, and weaponry. Most have specific requirements like 'reach wave six of battlefield X'. Kill as many enemies as quickly as possible with one shot to increase the Multiplier. As the Multiplier increases, you gain more Boins and that will let you upgrade your weapons. So play Forest Run to collect all the Boins. But be quick because the Boins will disappear after a while.