Halo Infinite

To get the achievement, launch (or restart) the Repository mission. Progress to the objective to activate the gravitational elevator. Two Chieftains equipped with turrets are blocking access to one of the energy cores. Eliminate one of them and use its turret to defeat the other.

Here is a video to help you:

Self explanatory, can be done on any difficulty and SOLO.

Boot up the mission the Repository, and progress through the mission until you reach the room with the second lift. You will not be able to progress through the second lift until you have 2 power seeds, the 2 chieftains are in one of the rooms containing one of the necessary power seeds. Once the lift room is clear, there will be 2 doorways on the bottom floor in the middle of the room that will unlock, take the one to the right if you are standing on the entrance to the room. At the end of the path there will be 2 chieftains each with turrets in their hands. Kill only one, and then use his turret to kill the other. If you accidentally kill both immediately unalive yourself with your grenades to go back to the previous checkpoint.

If you reach the end of the path and instead there is only 1 chieftain with a gravity hammer, then you have taken the wrong door. The 2 turret chieftains will be on the opposite door within the main room, simply backtrack and take the other door. If you intend on completing the mission, bring the power seed with you from the Hammer chieftain room to avoid unnecessary backtracking later. Hope this was helpful. :)

Edit: IceFrogger1313 indicated that they were able to get the achievement by bringing a turret with them from an earlier part of the level, and using it to kill one of the chieftains. Feel free to try it if you are feeling adventurous, and let me know if it does or does not work, thank you as always! :)

Start Repository mission, progress up the 1st lift, down a hallway (which has a squad in it). Then an opening with 2 brutes and down another hallway - "you found me" ( Cortana audio). The hallway opens into a room with the 2nd gravity lift that requires 2 power seeds. Clear the room and grab the plasma cannon between the power stations and go for the power seed to your left (facing away from the lift station).

If you went the correct way, there should be 2 chieftains in the room leading to a power seed. You only need to kill 1 chieftain with the plasma cannon you brought with you.

So here is a video guide done in English for those that prefer, Silver is one of the best at Halo...