on the 6th video down i found a different way of doing that. you have to be right across from the sleight of hand machine and double jump from there to the grenade spot. the zombies (if done correctly) will be stuck against the wall from where you jumped from. then if in a coop game have your other players stay in the power room and they can shoot through the window that the zombies are against. works great. i got to lvl 60 but then my friends ps3 lagged out so id didnt count