This battle was possibly the toughest I've ever encountered. Algalon has high health, hard-hitting, Elite legendary pets that had annoying abilities...so I wouldn't be surprised if a lot of people run into trouble with this battle.
I'm sure some will have no problem defeating him, but if you're like me and find yourself struggling against him, here is the strategy/pets that I used. (I wish I could battle him again so I could test out my strategy to make sure it wasn't just a fluke, but alas you can't fight him once you've defeated him once.)
Keep in mind that this is PTR, so Algalon's pets/battle is subject to change.
Macabre Marionette - H/P (1/2/1)
Emerald Proto-Whelp - P/P (1/1/2)
Infested Bear Cub - H/H (1/2/2) (I think H/P could work as well)
1.) Macabre Marionette - Siphon Life, Dead Man's Party, and then Macabre Maraca twice to finish off Comet.
Notes: The second Macabre Maraca should hit while you're on your Undead-round (when the marionette comes back to life).2.) Comet dies, Macabre Marionette dies. Swap to Emerald Proto-Whelp, Algalon swaps to Cosmos.
Comet CAN crit you. If that happens and he kills you before you can get your second Macabre Maraca off, restart the fight.
3.) Emerald Proto-Whelp - Emerald Dream, Ancient Blessing, and constantly use these two healing abilities whenever they're off cooldown while filling in the gaps with Breath until Cosmos dies.
Notes: You must time Emerald Dream just before Cosmos' force-swap ability. The bunny uses this ability on cooldown, which is annoying. However, when your pet is asleep, Algalon's pet cannot swap it out.4.) Cosmos dies, Emerald Proto-Whelp should still be alive, Algalon swaps to Constellatius
4b.) (Optional) Emerald Proto-Whelp - Ancient Blessing just for good measure (if available, not entirely necessary)
5.) Swap to Infested Bear Cub
6.) Infested Bear Cub - Use Hibernate on cooldown and just pass your turn until Constellatius kills all active pets with Apocalypse.
Notes: Because the Infested Bear Cub is Undead, it will take less damage from Constellatius' only attack (Tail Sweep), giving you more room for error.
Algalon's pet uses Wish pretty much on cooldown. Instead of trying to beat through the healing, use its Apocalypse ability against him. Outlast Constellatius by healing your pet and letting Apocalypse expire, killing all active pets.
Emerald Proto-Whelp should be last pet standing. Hooray, you've won!
I'm absolutely ecstatic that I finally defeated this encounter. I went through many different strategies and struggled so much; I thought that I would just have to wait until a pet battle-theorycrafter came up with a viable strategy.
But after consulting the OH and having him give it some attempts, we formulated a solid plan that involved knocking down Algalon's first pet (dragon) because it did massive AOE, avoiding getting force-swapped when facing his second pet (bunny), and then simply letting his third pet (hippo) kill itself. A huge thank you to the OH for helping me brainstorm and test different ideas. :)
Although it was initially VERY frustrating, it was ultimately VERY satisfying completing this Pet Battle. I can't wait to do it again on live once Patch 7.1 is released!
Last but not least, Stardust's idle animation - it turns into a shooting star, flies up into the air and circles around for a little bit before falling back down next to you. NEAT! :D