In the Fields of the Fallen you will find 3 NPC's spawn camping. After they kill the Monster you need to clear them out so you can get credit for the kill. If you do not, then they will get into a lengthy respawn. Complete with corpse retrieval! This is a great way to thin things so you do not have to fight all 3 at once.
For this one, you need to kill xxDatVaultHuntrxx then teabag him. AKA, crouch over his dead, bullet-ridden corpse. All of them are armored so be ready. Do this twice to complete the objective.