To successfully complete the Healer Gatekeeper challenge, be sure to include a self-cleanse such as Transfuse & Abuse.
To start the challenge, go to Agartha. From the faction capitals, move forward just past the Stationmaster and turn left into the portal there. From that hub, take the left branch which leads to the Gatekeeper (will later show portals to nightmare mode dungeons). In front of the golden Gatekeeper are 3 interactable branches. Interact with Vita for healing.
In An Egg Shell
Keep yourself and all Friends alive for the duration of the fight
Radiant Charge – Cleanse with Transfuse & Abuse
Abilities & Adds
Radiant Charge – Places 3 DoTs on you, must be cleansed
Radiant Burst – Melee knockback
When the encounter begins, "A Friend" will spawn who serves as your tank. Later more Friends will spawn as your DPS. It is your job to keep them all alive until the end of the fight. Because they do not show up in your party frames, you will need to manually target each one.
The Gatekeeper will place ground AoEs across platform, so you will need to use the outer edge. In addition, he will place point blank AoEs on your location every few seconds, requiring you to move, until about 50% health.
At this point, he will cast Radiant Charge which will place 3 DoTs on you. These must be cleansed with Transfuse & Abuse.
Around 30% health, 3 additional Friends will spawn to help DPS. However, 1 of the Friends likes to stand in fire, so alternate on healing the tank and the DPS in the fire. It will be difficult if not impossible to get them topped off, but just keep them alive long enough to kill the Gatekeeper.
At the very end, the boss will cast Radiant Burst which will knockback and damage the tank and melee DPS, so be prepared for it.
Defeating the Gatekeeper will award the Zuul achievement and allow you to begin running nightmare mode dungeons.