I thought Xurkitree was going to be the only one that I would have trouble with but Guzzlord was a pain too. Granted, my Pokemon weren't exactly sturdy but Thrash kept hurting my teammates. And yeah I like Celesteela too! It was hard figuring out the natures I wanted because it's so versatile in battle but I settled with Sassy and Quiet in the end!
Xurkitree was a pain to catch as I had to use my Toucannon as bait, switch to Kommo-o to deal some damage, switch to my Synchronize Espeon/Alakazam, and hoping that the Beast Ball worked. .__. And yes Pheromosa is pretty fantastic despite its small movepool! It helped me in the Battle Tree a lot!
[the adult fanbase in Pokemon is one that is very untapped - many of the adult fans of Pokemon, like us, have been with the franchise since they were kids, and so, an anime spin-off made for the older audience would be a nice way to tap into that market]
I actually had Tsareena as my temporary sixth member before swapping it out for Hackamo-o (which quickly evolved afterwards). Thanks to the new feature that allowed you to swap caught Pokemon in your team if it was full, I had a blast rotating Pokemon! It was pretty hard choosing a sixth member because I really love the Sun/Moon Pokemon a lot!