How to win: Do all the rescue missions. As soon as you can buy the auto shotgun. After completing all the rescue missions you should be able so fully perk your auto shotgun. On day 9 you can grind untill you have enough for the gauss shotgun. After you have that and the shotgun just rest every day he will let you.
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This game would be alot better if it was harder and had a final boss. even if on day 40 all the bosses would spawn with the rest of the zombies. 4/5 for fun, but 5/5 because it keep me entertained for 5+ hours