Ultimately, it doesn't matter that much. If executed correctly, the HPS of players should even out at the end of the encounter (and I've seen the healing meters spread as little as 5% between first and last).
Once you've gotten your assigned buffs, you'll want to switch to spells that really ramp up your healing per second (HPS). Every player is going to take damage. You can go easy during the opening minute, but Ultraxion's damage does ramp up as the encounter gets progressively longer. Use AoE healing as much as you can, as single-target spells simply will not be as efficient.
Players who take Hour of Twilight to ensure the Aspects aren't affected by them are going to need healing. Shadow priests should be OK, but I've found that I've had to heal both hunters and mages who took the bullet. Know the order of those if you can and drop preemptive healing spells (HoTs, for example) before you avoid the Hour of Twilight yourself.
Mana may become an issue toward the end of the encounter. I've found using cooldowns like Hymn of Hope or Potions of Concentration are best used during the red and green crystals. You should be using a Heroism or Bloodlust right when Nozdormu applies the Timeloop buff.
Speaking of cooldowns, have an idea of when you're going to use your raid-wide cooldowns as you get closer to the end. I'd suggest tapping any DPS defensive cooldowns. I'm sure your shadow priests and warriors would love to help here.
Atonement priests, maybe you guys can chip in here. I'm not quite sure which color buff is the most suitable. Could I ask for your thoughts in comments and what exactly you're doing in addition to repeatedly casting Smite (if anything)?
Warmaster Blackhorn and the Gunship
Once you're finished with Ultraxion, you'll get the opportunity to board the Alliance gunship the Skychaser and go after Deathwing. As you're busy pursuing him, your ship will be boarded by his Twilight loyalists before you engage the Warmaster himself.
When you're setting up, I suggest staggering healers around the main deck. Get them spread out, and have them heal players nearest to them.
Once you engage the Warmaster, you'll notice purple swirls on the ground. Your players need to stand in them. If unchecked, the ship will take full damage. What you're trying to do is catch them and soak the attacks so that your ship doesn't have to. So going against traditional convention, your raid group actually needs to stand in Twilight Onslaught. This ability is large, it's purple, and it swirls. You can not possibly miss it. Raidwide CDs should be used to mitigate these effects. It is absolutely critical that you heal the weakest players, or else the Twilight Onslaught ability will inflict lethal damage. Basically, this spell does an astronomical amount of damage divided by the number of players standing within it.
Meanwhile, the rest of your raid group needs to deal with the different boarding parties. Don't stand near them if you can help it. If there's no action going on, throw in a little DPS on the drakes or the big cultist guys on the deck.
After a few waves, Blackhorn himself will thump onto the main deck. Goriona will continue to blast the raid from overhead with Twilight Flames. Unlike the first part of the fight, you'll actually want to stand out of the fires. Your ranged players will be busy trying to fend her off, while the melee players will be working on the Warmaster.
Healing through Blackhorn can be a little tricky. Watch your distance and try to be over 10 yards away from him; otherwise, his Disrupting Shout will stop you from casting. At the same time, you don't want to be too far away, in the event of a Shockwave. If Shockwave happens to be dealt in your area, you need to be able to move really fast away before it actually hits, or else you'll be stunned.
All in all, I found the Warmaster Blackhorn encounter a little easier then I expected. You should be able to get past him fairly quickly.
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