Nazi zombies anyone?

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Has anyone went to more than level 25 in COD5 Nazi zombies? got a new strategy to share? here's what i do: I accumulate 3000 points as fast as i can and open the stairs up, then stairs down to help room and and spend the last points on a random weapon...after a while, the zombies kinda like come down the stairs in a single file so it's easy to score head shots and kill them fast, and i never open up the help room from the beginning room...but it becomes freakin hard after level 20..has anyone done this or have u got another strategy? oh and i always play solo in my pc,,,and been to level 25 max (without those stupid glitches, they ruin everything!)...ZOMBIES RULE!!!