If you are stuck i recommend to use this walkthrough = http://www.youtube.com/watch?v=yx_VUibFejI = ( It is very helpful on hardcore levels ) . Keep this comment alive to help people
There should be a zombie mode where 10-15 players are set randomly to zombie or human teams . There are more zombies than humans . Zombies have melee attack and turn the human into a zombie in 1 hit and the others try to survive.....Well at least that's all i wish