The hardest battles for me were against Totem Togedemaru in both of my Ultra Sun playthroughs. The funny thing is that I had a Ground type on both of my teams by that point (Sandygast on my first playthrough and traded in Golett with perfect IVs on my second playthrough) and I still had issues with Togedemaru.
Spiky Shield coupled with moves that can flinch you made this fight imo harder and more annoying that even the Ultra Necrozma one.