Achievement: Find The Yeti Zombie
It's easy, but you'll need to complete "Adventure Mode". After you complete the
adventure mode, you'll need to start playing through adventure mode again. When you
get to Level 4-10 (night time level, with that is mostly dark, but lit up by lightning
effects), you should get the Yeti Zombie eventually. It may take a couple tries, but he
should come out eventually. It's the only way to get the achievement.