To unlock this achievement, you must kill a Hunter this its own flechettes. To gain the ability to use the Hunter's flechettes against him, grab a large item that doesn't break, and hold it in front of you. Wait for the hunter to fire its flechettes. Some of them will attach to the item you are holding. Now, just launch the item that you are holding at the hunter. If the blast is powerful enough to kill the hunter, you will unlock the achievement. If not, wash, rinse, and repeat until you kill the hunter.