A Celestial Invitation

Took a few tries, but I figured out a strat that worked for me.

[npc=70451] - any speed - (2,2,2)
[npc=70154] - (1,2,2,)
[npc=88401] - (2,2,1)

Zandalari Anklerender opens
Primal Cry
Black Claw
Hunting Party (continues till death)
Zandalari Anklerender dies during hunting party
Swap in Direhorn Runt
Trihorn Charge
Comet dies, Cosmos comes out
Trihorn shield
Cosmos swaps in Son of Sethe
Use touch of the animus, plagued blood, and drain blood on cooldown to keep Son of Sethe alive
Make sure Cosmos has plagued blood and/or touch of the animus debuff when Direhorn Runt is forcibly swapped in
Cosmos swaps in Direhorn Runt
Stampede (Due to the debuffs on Cosmos this heals Direhorn Runt for a massive amount)
Trihorn charge/stampede until Cosmos is dead.
Constellatius comes in
Swap in Son of Sethe
Son of sethe is undead type, meaning Constellatius' only damage attack is weak against it
You can easily keep Son of Sethe alive using his 3 heal abilities until apocalypse triggers, killing both pets, reanimating Son of Sethe, and winning the battle.

Hope this helps, enjoy your new spectral bunny!

Ashamed it needs to be said, but the Algalon you need to battle is not inside the instance. He's located at the bottom of the stairs leading up to the entrance of Ulduar. Don't fly right by him like I did.

Fully AH-purchasable team:

1. 
   
2. Zandalari Anklerenderer (hunting party, primal cry, black claw)
3. 
   
4. Mechanical Pandaren Dragonling (breath, doesn't matter, decoy)
5. 
   
6. Grotesque (shadow shock, agony, stone form)
7. 
   

Zandalari: 2 => 3 => 1 => (zandalari dead)
Grotesque: 1 => (first pet dead)
Grotesque: 2 => GET SWAPPED
MPD: 3 => spam 1 => GET SWAPPED
Grotesque: 2 => 3 => 2? => GET SWAPPED
MPD: 3 => spam 1 => (second pet dead, possible dragonling too, finish off quickly with grotesque if that's the case)
Grotesque: 3 when below 1300 health => (third pet dead to apocalypse)

This team's win seemed very solid as each MPD and Grotesque received multiple crits and delivered no crits.