The gameplay mechanics and goals changed a bit like, instead of having missions, you only have to survive wave after wave of zombies. You will earn money depending on how well you performed and spend your earnings on upgrades and items at the end of each level. Anyways, you are not here for some review, I know right? You are here for the Mass Mayhem Zombie Apocalypse cheats so here is a list of codes and their corresponding effects to the game.
LOCKNLOAD - All weapons
BIGPOPA - Invincible
SURRENDER - Infinity off bullets
CHECKY - Unlock all stages
DARIDE - All medals + Stages Extra
COMEON - Doors reinforced
All of those cheats will sure get you ahead of the game but Using the codes above can ruin the excitement of the game so use it at your own risk. Meanwhile, here is a video of me demonstrating the Mass Mayhem Extra Bloody Zombie Apocalypse cheats (sorry for the poor quality, just giving you an idea where to enter it).