For this achievement you have to kill the invisible Elite at the beginning of Winter Contingency. This must be done on Legendary difficulty. As soon as you exit the dropship you'll need to sprint down towards the buildings. If you pay close enough attention you'll see a cloaked elite with an illuminated square on his back running to the right. Use your pistol to take him out.
It took me many attempts, but luckily a checkpoint triggers as he starts running away so you can reload that if he takes the corner and disappears. I used my pistol as I found the scope helped and the AR wasn't reliable at the range you have to start engaging the Elite at. Once you successfully kill the Elite you'll pop this achievement.
The glowing square that was on his back was actually a datapad that only spawns in Legendary. It should have dropped near where you killed him. Read it to pop another achievement.