Okay, so I finally got it down. It’s not easy, and none of the old strategies work anymore. None of them. I tried them ALL. So save yourself the headache. For real. Here is the strategy I used. It worked the first time I used it, but your mileage may vary if you don’t get crits in the right places.
Pet #1 - Zandalari Kneebiter (P/P)
You can substitute any of the Zandalari raptor pets with the P/P breed. My Anklerender is a S/S, and I had zero success with him. I am more than 50 attempts into this fight, so trust me: use the one that has a power breed, whichever one it is that you have.
You want Black Claw and Hunting Party. The other ability will not be used.
Death Seeker (H/H from layer two of Twisting Corridors) might work, as well, but I never got a crit from him, and it was always close, but never quite got there.
Pet #2 - Emerald Proto-Whelp (P/P)
Set your abilities to Breath, Ancient Blessing, and Emerald Dream
I don’t know if the P/S breed will work or not. I only have the P/P so did not try with the other breed. Know that using the P/S breed will NOT make you faster than Cosmos the rabbit, so it will not matter for the purposes of avoiding swap mechanic.
Pet #3 - Darkmoon Zeppelin (H/B)
Many online strats say you should use the Mechanical Pandaren Dragonling in this space, but mine died every time. It doesn’t have enough hit points. The Zeppelin has more health than the MPD, so I tried it out and it worked my first attempt.
The only ability you will use is Decoy.
Use the pet you have with Decoy that has the highest amount of health.
Comet is a Dragon, and he hurts. Bring up your Zandalari Raptor and use Dark Claw followed by Hunting Party. Your raptor will be defeated after Hunting Party ends. IF you do not crit, which will get Comet down to around 200 HP, forfeit and start again. This could take a while (bring many bandages), but that’s what the nerf to Black Claw has done to this encounter. I have tried EVERY hard-hitting pet in my arsenal to kill Comet, and this is the only one that works. Death Seeker getting a crit might work, as well, but it never happened for me.
Bring in your Emerald Proto-Whelp and cast Breath. This should kill Comet.
If Comet does not die after the first breath, you will probably fail the fight, but I have successfully gotten into the proper rotation on Cosmos with two breaths to kill Comet…so stick it through and see.
Cosmos is a Magic class. The mechanic you have to watch for is the forced swap he does. It’s called Phase Punch, and you cannot avoid it. Prior to BfA, people could bypass this swap, but that no longer works.
Use Emerald Dream and follow that up with Ancient Blessing. Never deviate from this even if your whelp is at 100% as the round begins. Cosmos will ALWAYS go first, and so you must use Emerald Dream followed by Ancient Blessing every time they are up, in that order. Breath until forced to swap.
Have your Zeppelin cast Decoy, and then manually swap back to the Emerald Proto-Whelp on the next turn.
Emerald Dream/Ancient Blessing/Breath rotation until forced to swap the second time. Decoy will be back up. Cast Decoy and manually swap the second time.
Repeat your whelp’s rotation until Cosmos is defeated. You should not be swapped a third time. If you are, and if the Zeppelin is defeated, you must forfeit and begin again. The Zeppelin MUST BE ALIVE and in the backline when the third pet comes out.
The final enemy pet is a Magic class celestial boar. You will not kill him. The idea is to stay alive long enough to trigger his Apocalypse. If your zeppelin is in the backline and still alive when Constellantius comes out, you will win.
Still on the whelp, use the same rotation and just stay alive for the 15 rounds until Constellantius is defeated by his own Apocalypse.