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Old things! Sash in Battle Stadium (3v3) is the most annoying thing ever, and so is Mimikyu! Fairy and Ghost have a lot of good options, but the fake Pikaclone is very likely to be in every team of its type. Watch out for Tera-boosted Shadow Sneaks.
Gallade, a good option for Psyhic, has a new ability, which boosts its "cutting" moves by 1.5 if I'm not mistaken. Night Slash, Leaf Blade, Sacred Sword, Psycho Cut, all get even better!
Dragapult is another popular Battle Stadium mon, so expect to see him in its types. People are crazy enough to run Scarf max speed in order to outspeed and kill fellow stealth planes.