Clear and Present Danger Achievement in Halo: The Master Chief Collection

This achievement can only be earned on the Legendary difficulty on Winter Contingency.

At the very start of the mission, after exiting the Falcon and gaining control of your character, you will be tasked with making your way down the hill toward the distress signal. As you're moving down the hill, you should notice the faint outline of a cloaked Elite running away. Quickly swap to your Magnum to take him out from a distance and unlock this achievement.

NOTE: It's worth noting that, despite sounding very easy, this may actually take several attempts as the Elite has somewhat of a headstart on you in terms of distance and then disappears after turning the first corner where two Moa begin to flee. With even patience, though, you should be able to kill him and collect the Data Pad on his corpse for the "Unreliable Narrator" achievement.