I’m sorry; I would go and find a new strat if I could. But having done it, it won’t appear again for me. And of course neither will it work for the other regular strat setters who have done it before either. 
Did you try the newest strat on Wowhead:
After some trial and error this strategy worked after SL pre-patch changes to pet battles.
The only harder to get pet is Son of Sethe - from Arakkoa Outcasts rep.
Chitterspine Skitterling (S/S) (-/2/2)
Infected Squirrel (P/H) (2/2/1)
Son of Sethe (P/S) (1/2/1)
Against Comet - Start with Chitterspine Skitterling - Black Claw , Swarm
Against Cosmos - use Black Claw (Skitterling will die), follow with Squirrel - Creeping Fungus, Stampede, Consume, Creeping Fungus, Stampede (both pets should die together)
Against Constellatius - rotate on cd Touch of the Animus, Drain Blood, fill with Absorb and wait till Apocalypse hits. You will win on death round.
Both an S/S Chitterspine and a H/P Infected Squirrel are easy to capture. Son of Sethe costs at the AH, but you might find another third pet that might work?
I also see another strat in the Discord, with a similar idea:
Here’s what I used. Chitterspine skitterling 2,2,2 Scabby 1,2,1 Infested Bear Cub 1,2,1
1 black claw
2-4 swarm
4 Comet dies Cosmos Comes out
5-6 swarm Cosmos kills skitterling and swaps to Scabby for you
7 Rabid Strike
8 Diseased bite
9 Hibernate
basically continue this until Scabby dies just make sure you hibernate before he casts the phase whatever that swaps you out
Bring in infested bear cub hibernate on cool down and use your other abilities to take down cosmos from here on out
just use hibernate on CD and wait for the apoc to hit…you win because you are an UD pet.
Chitterspine was s/s it’s the only way to make sure he kills the first pet with no backline support