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 (720NoScope)Headshotz by getting a headshot or critical hit. This one feels much more luck-based than any other. If you have a corrosive-type Sniper Rifle then it gets a bit easier at least. All of them are armored. This one easily takes the longest to pull off.