Can't touch this 20
Use a hammer to kill a series of 15 zombies without taking damage.
Here is a Video Done by Stallion83
Music MC Hammer "U Can't Touch This
Find a hammer or have a friend give you his in a coop game. Make sure to upgrade it to max and have it fully repaired before attempting the above. You cannot take any damage during the 15 kills.
Kick is your best friend (LB) use it to keep enemies off of you and to weaken them. Be sure the final blow comes from dropping the hammer down.