there are many different options for the trainers, though some require a little more strategy then others... there was a post combining them all, i think, so if you really want all the options i'd suggest looking for that.
Like the previous poster, i'd highly suggest leveling chrominus (you already have a water spirit at 25) you don't even need an unborn valk for that team to take out most celestials easy (geiser+whirphool, swap to chrominus, howl--> surge of power will eat xufu and chichi. for nizao, use howl,bite,surge of power instead after the swap). you have a gregarious grell at 25 which will easily take care of yu'la by himself(1,2,1, make sure to phase shift to avoid her liftof coming down)
for the earlier trainers, these were my teams:(next weeks wise mari, kyrin and blingtron give me the most issues, so beware that week might be difficult)
wise mari: crow(open with peck, then call darkness), dragonling(heal+tailswipe thill dead), crab(any, just keep attacking)
kyrin: restless shadeling, ragnaros, darkmoon zeppelin
blingtron: any water strider, electrified razortooth,fell flame.
sully: any frog, alpine fox or other beast with howl, any moth.
dr goldbloom:azure whelp, clockwork gnome (1,1,1), restless shadeling
lorewalker Cho: celestial dragon, any spider, scourged whelp
tharan zhu:unborn valk, S/S moth, another moth(S/S is best, but doesnt have to be)
chen: clockwork gnome, zandalari raptor, snarli(or any other crocolisk, really)
wrathion: lil ragnaros and scourged whelpling, any other pet. keep using their aoe abilities.
because the tournament is pve and everything is rather scripted, learning what the opponents pets are going to do next is usefull. Also, because most of the fights are really 3times 1v1, you can mix and match with other combinations you find for a team you do have the pets for already. Don't be discouraged! When this place just came out, it took me several days to figure out what teams worked for me aswell