I think it might be useful to have multiple ways of dealing with each of the celestial pets at the end, since we may or may not need some pets for the first 3 tamers that vary (although most people just seem to get by those tamers with a rock paper scissors match making system, which is fine).
I did mine last night and had to face Cho, Sully, and Ion.
Cho starts with Wisdom, a flying type with 1694 health, 300 power, and 338 speed (effectively 507 above 50% health). He can be fairly easily countered by a dragon type with magic abilities, bonus points if you have an ability that will allow you to avoid attacks, I ended up using my Emerald Proto-whelp. Patience will start off with wild magic, and you should start with Proto Strike. That will put you in the air to avoid the 2nd round rake (the from it hurts when combined with wild magic). After that you can just emerald bite and use ancient blessing to heal. I think Ancient Blessing is more useful than Emerald Presence here.
Patience, a magic type with 1928 health, 319 power and 272 speed is out next. Mechanical Pandaren Dragonling works well here. He has a dragon type attack, he's faster, and he'll take less damage from the sweep while not taking more damage from the humanoid broom attack which kinda whomped on the dragon I used (dark whelpling iirc). Shouldn't take much more than spamming your breath attack with the mechanical pandaren dragonling to finish off patience. Ideally you want decoy out for the start of Knowledge.
Knowledge is Cho's last pet, a dragon type with 1694 health, 375 power, and 263 speed. I forget if he opened with amplify magic and then solar beam or if he solar beamed by right of the bat. Either way it hurt like hell, but he has to recharge for two rounds so meh. I ended up killing him with my Kun lai runt, although it was a close thing iirc. I wanted to save my Flayer Youngling for Yu-la, but I shoulda just used it here since I didn't end up using it on Yu-la anyway.
I did Sully next. His three pets in order are:
Socks, undead type, 1975 health, 319 power, 263 speed. He has a beast type attack, so don't use a critter like I did. I spent a fair bit of time dying to socks and monte b/c apparently I've forgotten the standard undead counter team of three frogs I built for Nearly Headless Jacob. Sully's only got one undead, so one frog is enough. Mojo was able to kill socks and get some decent damage on monte, the next pet.
Monte, critter type, 1694 health, 375 power, 281 speed. He's got burrow, so something that can dodge is nice. He has Vicious streak which will boost his speed by 100% so you're probably not going to out speed him. Either way any old random beast type should work fine. I used one of my many zandalari raptors.
Rikki, aquatic type, 1741 health, 319 power, 309 speed. He's an aquatic type with beast moves, so any flyer with flying type moves is a super counter. The cute face ability is annoying but nothing you can't power through. I got my yellow moth killed on an earlier attempt on this guy so I ended up using my vampiric batling. I seem to be a bit short of flying types in my stable of level 25's. A nether ray free might be a decent choice here since you could equip it with bite to finish off monte in the event something goes wrong and then still have slicing winds to use on rikki, but I hate the randomness of slicing winds.
Last was Ion Goldbloom.
Screamer starts off first. 1694 health, 366 power, 272 speed (408 above 50%). It's a flyer with flying moves so grab your trusty dragon with magic attacks of choice and beat him down. Emerald Proto Whelp (if you didn't use it for cho), sprite darter hatchling, nether faerie dragon, emerald whelpling, w/e floats your boats. He'll end up starting with Lift off so keep that in mind if you have an avoidance ability that doesn't automatically go first in a round. If you don't want to deal with the Feign Death you could always howl+surge of power one shot him if you like.
If you let him feign, you'll get trike next (he's got more health than chaos). Trike a direhorn beast type with 1881 health, 333 power, and 267 speed. He's got beast and critter type attacks but no tri horn charge. He does have Adrenaline Rush to boost his speed to enable him to use horn attack. Bring a mech and beat him down, there's nothing really special here.
Eventually, possibly sooner than later you'll get to Chaos a magic type with 1694 health, 300 power, and 338 speed. He's got a humanoid ability that'll hurt your standard dragon type (whose dragon moves will do more damage to him since he's magic). But since he's magic he's capped at 35% of his health and I found I was getting to that cap with my high damage dragon moves so I'd suggest a beast since they'll take less damage from Logic. Alternatively something like a Sen'jin Fetish or Voodoo Figurine would have a dragonkin type attack without being a dragonkin themselves and getting their asses kicked. Same goes for Nether Ray Fry or Fledgeling Nether Ray.
On to the Celestials:
I did Yu-la first since she seemed the simplest. Dragon type, 2303 health, 591 power, 272 speed. Like Ti'un she's got a shield which combined with her -50% damage taken buff can be annoying but as long as you avoid multi hit attacks you should be ok. I used a feral vermling, my anubisath idol, and the third pet didn't join the fight although I think I threw in the Kun-lai runt since he'd survived the fight with Cho relatively unscathed. In hindsight I shoulda lead with the idol (led with vermling instead) since he coulda dodged the lift off. Nothing special heal really just beat yula down with humanoid attacks, it's 3 on 1 it shouldn't be hard unless you used up your good humanoids in the first 3 trainers.
Beast type, 2209 health, 606 power, 296 speed. I took advantage of the fact that blizzard seems to have a strange idea on how 100% damage buffs and 50% damage reductions should interact. I used a P/P zandalari raptor that accomplished very little due to being slower, but my Chrominius and a random mech, I forget which one, were enough to finish it off. The mech weakened him, then chrominius finished him off with howl+surge of power even with the whole "You can't take more than 35% of your health in one hit".
Beast type, 2303 health, 591 power, 294 speed. I'm not too sure what's wrong with this guy but last I checked 50% of 2303 is not like 1200 or 1300 that his wish healed him for. I don't quite remember if I managed to do enough damage to overcome the wish or if I died and tried again with a better set up/timing, but iirc I just pounded him with mechanicals like Darkmoon Tonk, Warbot, and a menagerie custodian.
Flying type, 2303 health, 591 power, 295 speed (almost 450 if she's above half health). I used a nexus whelpling with arcane storm and mana surge, followed by a shimmering wyrmling with mana surge, followed by some other random magic dealer to clean up. I may have gotten lucky in that she only tranq'd once.