Halo 5: Guardians

This achievement must be done on the Heroic difficulty level or higher. While it can be done in a solo playthrough of the mission, such a feat will require too reliance on the AI. Thus, it's easier to do in cooperative play with at least one other player. Both methods of play will be covered below however.

After defeating the last of the Soldier Snipers and Knight Commanders at the far end of the bridge towards the end of the mission, stock up on all of their weapons. Pick up both an Incineration Cannon and Binary Rifle. If you're playing cooperatively, your partner should do the same. If you're playing on your own, command your AI to pick up any spare Incineration Cannons. Now head into the chamber that houses the Warden Eternal boss battle and focus all of your fire on ONE WARDEN only. Kill him, try to leave the other two undamaged, then run around the perimeter of the chamber to trigger a checkpoint. If you get the checkpoint, create a manual save so you can keep that checkpoint just in case something goes wrong and you need to restart!

Take note that this chamber also houses supplemental Incineration Cannon ammo. On either side of the room, there is one cache with a Cannon, and on the far side of the room opposite the entrance, there are an additional two Cannons.

Now, at this point there will remain two more Wardens. This is what you'll want to keep in mind, whether you're in a solo or Co-Op playthrough:

• Each time you "injure" a Warden, he will drop to one knee and let out a scream
• The Warden can be injured up to four times; by the fifth time you "injure" him, he will die
• You can either injure him by shooting Binary Rifle or Light Rifle shots at the head or Incineration Cannon shots to the body
• After they are injured, there is a small window of immunity before they can take damage again
  • You need to wait until the Warden finishes yelling and the dark black hole in his center mass disappears
• A Warden's final death animation lasts about 2-3 seconds
• For this achievement, both Wardens must be in their death animation at the same time

With all of that in mind, you should then follow these steps to get this achievement done without much trouble:

1. If playing Solo, mark one Warden so that your AI can use their Incineration Cannons to injure the target (AI have unlimited ammo); once the Warden has been injured 4 times, repeat this tactic on the second Warden (you can obviously shoot as well to help)
   • Once both Wardens are one "injury" away from death, mark one Warden and allow your AI teammates to shoot him; it should kill him and initiate the death animation
   • Now quickly turn your attention to the second Warden and fire off the final killing blow, and if timed correctly, both will be in their death animations
2. If playing Cooperatively, ignore the AI and coordinate with your partner to get both Wardens down to their last "injure-state"
   • Once both Wardens are one "injury" away from death, communicate with your partner and then simultaneously attack each Warden with one last injury-inducing blow
   • If timed correctly, both Wardens will get caught in their death animations

All in all, once the remaining two Wardens are caught in their death animations simultaneously, the achievement will unlock! It may require a ton of checkpoint reverts, but with enough persistence, this can be done! It may seem extremely confusing when read through text, so if a visual representation of how this is done is required or even preferred, please refer to video guide below. Thanks for the video footage, Maka!