You unlock seven days of hell after you beat the game.... be warned if you beat the game in multiplayer the host is the only one who unlocks the mode. once you have the mode unlocked from day one to day seven is about 18,200 zombies. the whole game is easy because you have unlimited continues. you can unlock the achievement by joining someones game that has already beaten it and start from day 7. you will only have to kill about 3800 zombies that way. it for sure works.