I think for an average player even the easy difficulty is impossible (I'm stuck on stage 10-2). I wouldn't recommend this game for easy achievements, even though there are some of them.

Don't worry normal at 12-4 becomes impossible as in no one can do it without saving and exiting game.  Expert, forget it.  Same thing.  The challenge is absolutely impossible and would take quite some time to save it. 

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Zombies everywhere, zombies everywhere, zombies everywhere. They're coming for your brains and it's too late to change.