**UPDATE--Still works 2-11-2017
**Solution updated September '16 due to major updates. NOTE: The new requirement for this achievement is to "complete a Void Fissure mission with an enemy level of 30 or higher with only headshots." It is confirmed that the same method used previously for T4's works still! However, it does NOT work in Void Fissure missions for some reason.
Having helped others get this many times now, here's the easiest way to get the achievement
1. The host can't get the achievement this way, so take turns.
2. Guest who needs the achievement joins Host in a lobby.
3. Host starts up Void node Oxomoco(thanks Morf55!).
4. Host gets 1 headshot then stops killing(confirm the headshot worked, in the progress menu!) -- If the headshot didn't count, then abort/retry.
3. Guest kills everything else then dashboards.
4. Guest rejoins then both players extract.
*HIGHLY RECOMMENDED: Guest rejoins and dashboards again before extracting just to make sure the stats are erased!*
Note: If you are having the rare trouble of getting Host to stay host due to forced host migration to Guest at the start, then have Host start the mission, not kill anything, and wait for Guest(s) to join. (Credit to Humpyrdon)
This takes advantage of the games auto-rejoin feature which will give you the option to rejoin whichever game you were just in. It's very handy if you get disconnected legitimately, and in this case, VERY handy if you disconnect intentionally!
Guest needs to be powerful enough in order to kill the level 30-35 enemies.
Hope this helps!
Host can shoot multiple times, and the same enemy more than once--the only thing that matters is that the number of kills and headshots are exactly the same.