Alternate reload - Tap and hold the ammunition clip with the stylus. While that is held, tap and hold the left corner where the ammunition is supposed to be dropped with your finger. Lift up the stylus and the ammo clip will instantly appear where your finger is held on the touch screen. Lift up your finger and you will reload.
Completing Chapter 3 - When you get down into the tunnels, use the crowbar on the zombies instead of your guns. Once you reach the part in the tunnels when the bats fly at you, use your hand gun on them. After that, use a combination of your shotgun and hand gun on the zombies. If you can do this successfully, you can make it out of the tunnels and complete the chapter. There will be more zombies outside the tunnels for awhile; make sure you are ready for them.
Defeating swamp zombies - These zombies are smarter than the others. They will protect their heads with their arms. Use your hand gun on their legs when they are at a good distance, but still fairly close. You might have to shoot in quick succession for it to work effectively. Alternately, just blow their legs off with the shotgun. Be careful with your ammunition as you will need it for the Boss at the end of the swamps.
Furious mode - Successfully complete the game in Normal mode to unlock Furious mode.
God mode - Successfully complete the game in Furious mode to unlock God mode.