Here is the full list of all 18 Hillbilly Doomsday trophies - there are 1 bronze, 8 silver, 8 gold and 1 platinum.
Good job! Completed all tasks!
Defeat Aliens
Complete game
Defeat Zombactor
Get RPG
Defeat Zombrain
Kill 1000 enemies
Buy second pistol
Buy micro SMG
Buy flamethrower
Buy shotgun
Buy minigun
Defeat Zomsquad
Kill 500 enemies
Defeat Acid Ghoul
Buy magnet
Defeat Sterozombie
Kill 100 enemies