During Chapter 3 you are required to clear out the enemies and get a gate open for the Doctor. Once you have cleared out the enemies you need a chainsaw to get the gate open and you have to get this off the Sadist who is trapped inside the barn. If you look around enough you will find explosive bolts and a crossbow which will help you kill him.
To get him to appear you need to walk up to the area he is trapped in and he'll break free and begin chasing you. So equip the crossbow and shoot him straight away with an explosive bolt and then make your way out of the barn and to the left and go left of the gate into the door, there will be a trap on the floor and as he's following you, run up the stairs and trigger the trap and this will kill him netting you the achievement.