For people having trouble with the zombie achievement who are playing this game just for the achievements and don't really know the controls, press "z" on that level. That switches to the shotgun, which is really effective when the zombies are lined up against your wall.
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 "Massive Mayhem 3"){kind=icon}
 "Upgrade Complete 2"){kind=icon}
 "Storm the House 3"){kind=icon}
 "Gravitee Wars"){kind=icon}