Wow, this is tough
Yeah. When I first played the game, the Totem Araquanid, Totem Kommo-o, and Ultra Necrozma kicked my ass when I first fought them. It was my second attempts at them that did it in my first play through of Ultra Sun.
I just now saw it.
Yeah. That Kommo-o almost made me lose the first time I fought it again. And my star player for that boss was Metagross.