Aggapuffin

There's a Mystery Egg that you can get in the Old Woodland by giving someone a Fletchling Egg that has a pretty small chance to hatch into a Ralts. If you don't get it there, then you can get one when you get access to Cellia East through the Credit Store. Cellia East isn't unlocked for a good while, though, so don't hold out for Ralts if you don't get it through the Mystery Egg.

The level cap is 30, just one off of 31, so Sewaddle wouldn't be able to learn Sticky Web until after the gym, but it would really matter since Flying Type Pokémon are immune to Sticky Web. Just in case you haven't already beaten it yet, looking at these Pokémon, my first observation is Burmy, as, if Desolation has the level-up movesets from Gen 7, its evolutions are the only 'mon here that have Quiver Dance by Level 30. I would recommend specifically Trash Cloak Wormadam, as it's the only one of the Burmy evolutions that resist Gligar's Aerial Ace. If you can set-up 2 Quiver Dance's in front of it, you could probably sweep from there. If you can't set-up enough Quiver Dances, you could also try using Reflect from Ledian to make Wormadam even more bulky. I'd recommend keeping Protect as well to try and stall out Tailwind if necessary. Wormadam is comically slow, so even at +6, if Tailwind is up, it's very likely for Wormadam to be out sped. Also, it might be required to do the most annoying thing possible, that being to get a Wormadam with a good Hidden Power. Wormadam's only special move at this point is Confusion, which, even at +6, might not be enough to KO Xatu. It might, try without it, but I'm just giving you all the options you might need here. EDIT: Just remembered that Gligar has Brick Break, so you can ignore the Reflect idea. Oops. God, I've been theory-crafting this for like an hour... what's wrong with me?