1.mi-go, mi-go is realy strong, fire is to weak, mele is nothing and guns without some skill are useless alslo have long sight (almost like player) and its fast (almost like player too) at least is still possible to run away if you do not have pain
2. moose, these blood thristy demons can and will chase you for untold miles of zombie flooded land
3.skeleton and skeleton dog, just hard to hit these things
4.greed, sometimes i can run in middle of horde just for few common things
5.boredom, last time i was safe in my lab with food and water supply for years (4 fridges full of meat, 2 30 galon barrels of clean water, some canned food and beer) and safehouse is in middle of zombie horde i was bored by waiting so i decided to go outside