Simply because it’s impossible to get in MP.
Thriller (medal): Kill 15 humans as the zombie without dying.
Number of players in lobby: 12
…derp
Mhhh…Brains (medal): Kill 10 humans as the zombie without dying.
Number of players in lobby: 12
Number of starting Zombies: 3
12-3=9
So 9 Humans in total in a starting game…
lolfail
Discuss and what medal do you think is the hardest to obtain (excluding the impossible ones)?