Amidst Ourselves

During the Infiltration of Dread phase when Kin'tessa hits 100 energy there will be a small box up on one of the ledges around the room with 4 lights above it. Clicking one of the lights will cause adjacent lights to flip on or off. Simply turn on all 4 lights on one of these boxes before the Infiltration ends and then kill the boss. Only one person needs to do it and you can do it in any of the Infiltration phases so if you miss it the first time just wait for another phase and do it then.

The boxes look like this.

Contact me if you need help with the achievement, always happy to help guide people!
Btag: FrostyShot#1667
Discord: Yarini#9999

Managed to solo this achievement in heroic mode (not sure if the difficulty matters).
Simply waited for Kin'tessa to hit 100 energy and the boxes appeared. First time I failed because I turned them all off (wasn't sure what I had to do). Next 100 energy they reappeared, hit them onto on, killed the boss and the achievement popped.

Do not do this solo on mythic, you will get [spell=363235] cast on you and become unkillable if geared and unable to move, completely incapacitating you for around 10 minutes until bosses enrage. Even then they had difficulty killing my character.

Update: tried speccing tank to see if I could avoid the debuff and did not work, PvP Trinket worked however I ended up immediately back under the horrified effect in seconds