Mission 14 - The Breaking
Difficulty: Heroic (Co-op Highly Recommended)
After progressing through the mission you'll end up in an area where 3 warden eternals spawns. Before hand make sure your entire team is well suited up with Binary Rifles & Incineration Cannons. If you're attempting this solo, you'll want to command the AI to pick these weapons up off the ground.
Once inside the area kill 1 of the wardens without damaging the other. Now run circles around the outside to trigger a checkpoint. Now coordinate the other 2 wardens to die at the same time by communicating the amount of damage being done with incineration cannons by seeing how many times you "stagger" the wardens. Try to have both wardens at the same stages of damage at all times. If on solo, command the AI to kill one of the wardens, and immediately kill yours as theirs goes down. If in co-op (recommended) just communicate and coordinate your shots to make sure they both go down at the same time. If yours dies first, quickly switch the the Binary Rifle and try to damage the other as quickly as possible. Both wardens must be in their death animation at the same time for the achievement to unlock. Quickly reload the checkpoint if you fail, and don't forget that you can save the checkpoint just in case you trigger the next one!