You need to hit a zombie with a melee weapon
and the when a
appears press it to insta kill em and do it 100 times
Note1: Turning on auto instakill in options works (My friend got it that way) then you do not have to hit
only
and it will instakill zombies when there else would be a
above their head
Note2: Sometimes doesn't work if playing co-op if not host (my friend was host and he got it but mine stayed at 11%)