So here is a pretty memeish core that can actually work decently well with the right teammates.
Gardevoir @ Salac Berry
Ability: Trace
EVs: 252 HP / 192 Def / 64 Spe
Bold Nature
IVs: 0 Atk
- Calm Mind
- Substitute
- Draining Kiss
- Stored Power
Latios (M) @ Colbur Berry
Ability: Levitate
EVs: 252 HP / 4 Def / 252 Spe
Timid Nature
IVs: 0 Atk
- Simple Beam
- Memento
- Draco Meteor
- Thunder Wave
Kommo-o @ Salac Berry
Ability: Bulletproof
EVs: 52 HP / 204 Atk / 252 Spe
Jolly Nature
- Substitute
- Belly Drum
- Drain Punch
- Poison Jab
Basically the idea of this core is to use Latios's very underrated access to (Meme)nto, and where usually some would (mostly me) use Reflect/Light Screen, It's even faster to just use Simple Beam into Memento. He has obligatory Draco Meteor to scare off Pokemon like Garchomp that otherwise take advantage of Simple, and can Thunder Wave targets to make sure Gardevoir or Kommo-o can sweep. There is also the surprise factor, as most players will just instantly switch into something like Slowking to minimize Draco Meteor damage.
The important part of this gimmick is letting Gardevoir trace Simple, as basically it's the only Pokemon that can really viable use the ability in single. With Calm Mind and Substitute while facing off against an opponent who has -4 Atk/SpA, you can pretty safely set up, then continue setting up because of Substitute's protection guarding against anything paring multi-hit attacks and duel attack links (and if they are special, Gardevoir basically takes 0 damage from most special attacks. After set up is complete, Gardevoir will ideally be left with +4 SpA, +4 SpD, and +2 Speed while having a sub up. The Speed EVs are there so that Gardevoir is actually slower than most opponents, making it so that Gardevoir replaces the Sub after it's gone, and then is fast enough to outspeed Brokenpult and Zeraora. After set up, you can Drain Kiss to heal off all the damage by Substitute, and a 300 BP Stored Power to sweep. You can use Moonblast over Drain Kiss for more immediate power, but you may be more vulnerable without Drain Kiss's healing.
Lastly, as a counterpart to deal with Rillaboom, Kommo-o has instant +6 Atk and +1 Spe with a pretty good typing that resists tons of priority. Double Priority being the bane of Gardevoir, while Kommo-o can shrug off the priority thanks to its bulk, typing, and having a Sub up usually. In fact, this is just an identical to the Belly Drum set in Gen 7, but with Poison Jab for Unaware Clefable instead of Thunder Punch for Toxapex (which is rare and you can use Gardevoir or weaken it enough to where it'll be forced to Regenerate Health, while its teammates are KO'd left and right). Coincidentally, +1 Kommo-o is just barely fast enough to outspeed Dragapult like Gardevoir can.
This core is fun to use, but does face some issues, namely every team needing to run strong priority, everything being fast, or Unaware Clefable with the Meta basically letting you use Any-Type Max Airstreams, and if a team only has 1, then this core can cheese its way to victory. Thankfully, there are 6 Pokemon on a team, so you can cover up the weaknesses of the core with support Pokemon.