This Trophy is for taking out a zombie's legs. The first chance you'll get to earn this is on the train. Defeat the scorpion boss without using up all of your hunting rifle ammo, then find a zombie. Let him get close, aim down, then fire. If it worked, one of his legs will be blasted off and he'll fall to the floor.
Works in tandem with:
*As an aside, it appears someone at Capcom is a fan of 311, going by the names of a couple of the Trophies in this game.