Plant TNT, a Teller mine, or a grenade on an enemy corpse and convince an enemy to come near it. If necessary, you can move a body into position before booby trapping it.
You can also booby trap a body, attract an enemy (by whistling, throwing an object, shooting, letting them spot you) and make a save as the enemy approaches. Then once your target sets off the booby trap and is killed, reload the save and repeat until you have 20 kills.