Itemization
For this fight, I am wearing Master Architect, Powerful Assault, the "Pearls of Ehlnofey" Mythic Item, and Symphony of Blades. Pearls allows the wearer to generate 5 ultimate on casting a healing ability whenever they are below 30% of their dominant resource, which is why my Magicka is so low all the time. Using Pearls, I am able to generate enough ultimate so that I and the 5 DDs in my stack have enough Major Slayer to last the entire duration of every parse phase- this is why I always use my Horn first.
Composition
There is also a Necromancer "support DD" who is utilizing a special ultimate generation build so that she can give her stack Major Slayer to last the whole of every boss phase (or very close to it) as well- she always uses her Colossus first.
I am using Powerful Assault so that the off-tank can wear two light armor sets (Olorime and Martial Knowledge), and therefore spec to do a bit more damage as a Magicka Warden DD. I am using Symphony of Blades for the three people not using the Ring of the Pale order- the support DD, off-tank, and main tank. After discussing with the group, we decided that other monster helms wouldn't have as great of an impact as Symphony to help those three people sustain.
On my role in general, since all but the support DD are using the Ring of the Pale Order and therefore cannot be healed by me, I do not have to place my Illustrious Healing in the group except for when the support DD gets the Flare (she does not stack in the Flare when she does not have it on her).
The rest of the time, I try to keep it on the main tank since he has the tough job of managing not only the boss, but the three Atronachs that spawn in as well. Buffs and debuffs that I try to keep up as best as possible are Minor Berserk, Powerful Assault, Elemental Drain, Minor Sorcery, and an infused Weakening Enchant on the boss. I am also guarding one Sorcerer DD so that he doesn't have to run Barbed Trap. I do not need to run Orbs since all of the DDs are using them, and the support DD is throwing her Orbs toward the main tank for Alkosh. When procing Powerful Assault, I try to get it on all of the DDs in each stack. Since it has a cap of 6 people per proc, I must try to outrange the DDs that I don't want to proc it on, which is why I am running all over the place.
I initially tried to use Vigor as my Assault Skill, which also works to proc Pearls, though found that I ran out of Stamina pretty quickly, which proved problematic with uptimes and survivability.
Speculation
In future runs I would be willing to try this again, though, since I initially used Vigor while also running PotL, which also drains stam. PA seems to be quite reluctant to proc on the DK and NB DDs we have in the group... I try to keep my Ritual down so that the DDs can use the synergy when needed, and so the main tank can take it to proc Alkosh and receive healing.
To ensure that the main tank can be standing in it at all times, I always try to place it centered around the "head stack" of DDs. Finally, to prevent the DDs from getting 1-shot by the light attacks from the Flame Atronachs as they spawn, I cast Bone Shield twice, once in each stack, so that every DD gets a shield on them.
So far this seems to be a reliable way to ensure everyone's safety. In future raids, I plan to swap Vamp Bane to my front bar and Bone Shield to my back bar.
This is because procing the Templar's "Prism" passive on a Decisive staff has a chance to give me extra ult. I also do not need to cast Bone Shield as frequently as Vamp Bane, which should make for fewer bar swaps. Additionally, I'd like to try to better prebuff PA before boss landing after every add phase.