Xerneas is Washed

Clickbait aside, many people are worried that Xerneas coming back in Pokemon Champions, especially with Tera, would be game breaking. Personally, I doubt Xerneas would be nearly as dominant in a Gen 9 format as it was in prior generations - I don't think Xerneas will be look that broken next to the Horses, Bikes, and Dogs of the latest generations.

Zacian in particular is a very good counter, even after getting nerfed. And while Tera works in Xerneas' favor, it also works against it - Caly-I, Zamazenta, or Koraidon could all tera to Steel/Poison/Fire and threaten Xern even after Geomancy. TR is also a more viable counter than it was before - Xerneas can't stop a Calyrex-Indeedee lead from going Follow Me + Trick Room. Sure, Xern might get boosted, but then gets threatened by a Helping Hand Glacial Lance or Iron Head. It survived and took a KO? Great, Amoonguss will come in with Clear Smog, or Kingambit with Iron Head, Ursaluna with Facade... you get it. We have better TR setters, and better slow Physical attackers that can turn Geomancy's speed boosts against it.

We've also got new options to counter it, like Psyshock Caly-S, variable speed making Prankster Tailwind viable, Ogerpon-H with Follow Me and a Fairy resist, Prankster Haze on Murkrow, Gholdengo, and Unaware Dondozo. No, none of these things will single-handedly beat it, but they provide a number of options to play around it while still remaining strong into the rest of the meta.

Yes, Xerneas would warp the meta around it, and teambuilders will adapt to counter any of these strategies - but not all of them at once. Like Wolfe said in a recent video, the issue wasn't countering Xerneas - it was countering Xerneas, and being strong into the rest of the field. Nowadays, I think we have enough options that Xerneas wouldn't be any more meta-warping than the Horses and Bikes are now. Sure, the Deer's damage output is still ridiculous - but she's not alone in the lead any more.

TL;DR:

Yes - +2 252+ SpA Fairy Aura Tera Fairy Xerneas Moonblast vs. 252 HP / 196 SpD Calyrex-Ice: 218-258 (105.3 - 124.6%) -- guaranteed OHKO

But consider - 252+ SpA Choice Specs Hadron Engine Tera Electric Miraidon Electro Drift vs. 252 HP / 196 SpD Calyrex-Ice in Electric Terrain: 230-272 (111.1 - 131.4%) -- guaranteed OHKO

And don't forget - +1 252 Atk Zacian-Crowned Behemoth Blade vs. 252 HP / 252 Def Xerneas: 234-276 (100.4 - 118.4%) -- guaranteed OHKO