Elio: ((Is thermally-protected and insulated via an extremely potent Aura-based epidermal layer stemming from years of Aura Training and thriving in a variety of harsh environments that are common to both the Black-Ringed Pocket Dimensions littering Ultra Space and the remote hazardous parts of various regions he's traveled to))

Dunno, just have Heim cool him off with a mitigated [Blizzard] or [Frost Breath] attack. Or he could just go for a swim - make a beach day of it. Probably buy some cold desserts for himself; a smoothie is one of his go-to's after all.

Stahl has an ice cold beer and air con on. Simple and fun.

Arryn is out in the sun soaking up all the heat, she does not cool down she will out shine the sun if need be.

Avian is suffering in silence.

Balei either goes to the beach to relax or has all the fans and ac in her house at full blast (Alola summers are torture lmao)

Fausto is eating 50 double pops, stick and all, and getting a tummy ache

Ice Cream, beach trips and hanging out on high roof tops helps Linn keep cool!
Silvana just turns on the AC and continues playing games. Lazy cat-

Perks of having ice type Pokémon. All she gotta do is nuzzle against her Sandslash with a parfait or some other cold treat

Jin grew up on a what was basically a tropical island so she’s used to dealing with the heat. When AC isn't an option it was a lot of playing in the water, keeping rags dipped in cold water on her head, and sometimes it just meant walking around in as little clothes as possible. That of course not being an option she could reliably use everywhere but around the house she's more likely to just lose the top before resorting to AC.
Mary meanwhile is both Galarian and wears long sleeves most of the time so she just melts without air conditioning /hj

Norra: Blast the AC while chilling at home playing Video Games.
Kazi: Stays at work, either helping a client with a Tattoo, sketching, or taking a nap in the back office's couch.
Eudoxia: She goes to the Spring deep within Kirvy Forest, and cools down there.

Odd, whenever it's slow at the shop (and you can convince him to step away from the air conditioned comfort of his apartment), likes to visit the beach when it's not too crowded; beyond a chance to cool off, the ocean also holds a lot of nostalgia for him since he grew up near it.

Tyler loves the Summer, but if he's in need of a cool down, he'll just fly over to the nearest source of water, like a lake or beach and just hop right in

That or he'd get ice cream or sit in the shade while fanning his Dragonite, while she fans him with her wings

He'll use the AC in the Pokemon Center if nothing else works, but he doesn't like to loiter about

Kingston: eat watermelon
Ruth: (lived in alola for most of her life and is used to the heat) she probably just takes a shower and leans on Mila (is a nice cold snake)