Kill a cloaked Shrouded enemy.

The Shrouded enemies can be found in the Dune Sea and the very Western part of the Broken Tract areas of the map.

The Shrouded will, on occasion use a cloak to go invisible for a moment while in combat and will do this regularly when you fight them. Though it can be entirely down to luck if you happen to kill a Shrouded enemy when they are cloaked, you can somewhat force this to happen. Kill all but 1 of the Shrouded in an area and then follow them around, they will eventually use their cloak for a second. You have to be fast to kill them, so I used the Rocket Launcher and just aimed at the ground beneath them. As soon as they used their cloak I fired at their feet, killing them.