I think we'll have to pay a little more attention to the various mechanics, especially the Ominous Crackle minibosses, than we do in LFR.
10-man comp: 2 tanks, 2 healers, 6 DPS.
25-man comp: 3 tanks, 6 healers, rest DPS.Boss is immobile and has the usual raid-wiping ability if no one is in melee range.
There's a pillar of light in the middle of the room where you fight this guy; if someone stands in that pillar, then (a) the Sha of Fear will fixate on them, and (b) a cone will project behind them that reduces their damage taken by 10%, provides immunity to fear effects, and provides immunity to Breath of Fear. He uses Breath of Fear about every 30 seconds; it's 300k shadow damage+5 second fear to everything on the platform. When he isn't casting this, it's quite safe to be outside the cone (so melee can stand behind him, if they like).
The boss will summon adds that cannot be attacked from the front; you'll have to run behind them to do anything to them, and naturally since they'll be facing towards the cone that means not being in the cone for a lot of the fight. Both the boss and the adds will be spamming little Shadow-Crash-looking projectiles on the ranged.
Every fourth swing the boss will Thrash, hitting three times. Tank damage might be a bit spiky as a result.
The other gimmick is Ominous Cackle, in which five players (a tank, a healer, 3 DPS) get sent to one of the outer shrines to beat up one of three identical minibosses. They have two abilities that you'll have to worry about : Dread Spray (shoots cones of arrows in the direction of random players; if you get hit twice you get Feared for 2 seconds) and Sha Globes, which get knocked out of him as he loses health and have to be picked up within 8 seconds or else he recovers them and regains health. When you pick up a globe, you take some damage but gain mana/rage/whatever. After the miniboss is beaten, a portal appears that sends you back to the main platform and gives you a damage/healing/movement speed buff for 30 seconds.
He casts Ominous Cackle about every 90 seconds (45 on 25-man); if the miniboss group isn't back on the main platform by then, he'll send his current tank over and welp.
Reading through the comments on Icy-Veins reveals one big thing that we could do to improve, which is try to have as many people as possible stand-in for death blossom. Of course, the tank should save their biggest CD for Death Blossom as we were doing. But beyond that, several classes have personal cooldowns that will allow them to stand in and our healers also have cooldowns that should be used here. Several people pointed out that paladins can use Hand of Protection on a caster to allow them to stand in (requiring good communication in advance about who that will be). A druid healer could possibly use Tranquility to allow everyone to stand in. A priest could use their bubble. In addition, all dps should review their talents and glyphs to set up a physical damage CD that will allow them to stand in for the blossom. This should easily buy us the 3-5 seconds that we were needing to win this fight.
On a separate note, abilities such as tremor totem and fear ward should be used during the dread spray, along with cds that reduce or prevent shadow or magic dmg.
Finally, people returning from the platform have 60% damage increase and immunity to fears which means they won't be cc'd by Breath of Fear. I'm not sure whether they will still get the damage from it. We should spend one pull testing this out. If they don't get the damage then this is obviously an option to have some dps go straight to the adds and clean them up while ignoring the breath of fear. Even if they do, some classes have good ability to avoid shadow damage that they could use here.
Looking at the logs shows that we don't take damage while buffed with Fearless and can stay out until it wears off and the next cast. Use that time to take adds down and get some DPS on Sha.
Note that logging and meters are a little funky here, the Panda minibosses are far enough away to miss events in the combat log.
Thanks, Aanvil. That meshes with what I was thinking. Specifically, improving our performance on the Pandaren with Crossbows will make a big impact on the fight. Not just in the "staying alive" part of always having a tank on the boss, but in the "killing the boss" part. We're managing to stay mostly caught up on Terror adds until someone dies while "away" or the group doesn't get back in time. If we can kill the Pandas 10-15 seconds faster, that will be a noticeable DPS boost. If three DPS can mostly keep the Terrors under control, then six will wipe them out and/or get DPS on the Sha.
The first key is making sure we grab the health globes. If just two get back then he's gained 10% which is getting pretty hefty. We suggested spreading out and taking care of your slice, but we did not seem to be doing well at that. We tended to overlap a lot. Some really good suggestions for Dread Blossom here. Tanks should definitely stay in and grab globes. Using Hand of Protection is a spectacular idea! Obviously on a caster, not on a melee or me. Even if you can't DPS during it, just picking up the globes is helpful. It's easy for them to absorb two or three globes during a Blossom.
Here is an add-on that may help with the Sprays too. Since completely dodging the Sprays suggests hanging out in a couple of specific spots, grabbing the globes will become rough. So it seems more important to make sure we get those, even if we get feared a couple times.
It might be worth waiting to cast Heroism shortly before the first Cackle. Obviously less boss damage, but taking down the first Panda quickly may be worth it. Also, the fight could easily go over 10 minutes, so we should be able to get two Heroisms in! That's nuts.
For Science!