Archon's Sword is an elite kett soldier encountered during the final phase of The Journey to Meridian, one of the final missions found in game. The task has led you to the station's command hub, which is also the background to the encounter.
The Archon's Sword is a more powerful version of an Anointed, which means that he is also equipped with an ability to apply a blue cloud which cloaks himself and his allies. If you weren't able to distract the opponent or he has disappeared from your field of vision and managed to apply the cloak, consider throwing a few grenades or area of effect abilities to "survey" the area. To avoid any unpleasantries, you can also plant a few mines or other traps on the walkways leading to your initial location.
Be wary that you are standing before a prolonged encounter - each time you've managed to destroy his shields and began to deplete his health points, the Sword will retreat to the safety of the hall and send additional units to engage your party. In that case, switch your focus to the remaining kett and eliminate as many of them as you can. Take care of the opponents next to the boss firsthand, as they may receive the cloaking and disappear from your field of vision.
The further waves will consist of more powerful enemies, with the final one being joined by a Fiend, who should be taken care of firsthand. Stay mobile and retreat behind the structure if you are short on ammo or health. Finally, your opponent will run out of reinforcements, and will reenter the battlefield. Simply eliminate him by depleting his health bar, as this time he won't be able to retreat.