Bronzong @ Leftovers
Ability: Levitate
EVs: 252 HP / 252 Def / 4 SpD
Relaxed Nature
- Stealth Rock
- Gyro Ball
- Earthquake
- Hidden Power Ice
Tapu Fini @ Leftovers
Ability: Misty Surge
EVs: 248 HP / 192 Def / 16 SpD / 52 Spe
Calm Nature
- Taunt
- Nature's Madness
- Scald / Moonblast
- Defog
Ability: Levitate
EVs: 252 HP / 252 Def / 4 SpD
Relaxed Nature
- Stealth Rock
- Gyro Ball
- Earthquake
- Hidden Power Ice
Tapu Fini @ Leftovers
Ability: Misty Surge
EVs: 248 HP / 192 Def / 16 SpD / 52 Spe
Calm Nature
- Taunt
- Nature's Madness
- Scald / Moonblast
- Defog
I'm pretty surprised that this core wasn't mentioned already, but better now than never.
The rise of many SM powerhouses like Ash-Greninja, M-Metagross and Tapu Lele has impacted the meta about as gently as a roadside robbery. If you're like me, and can't play offense worth a damn, then Bronzong is the choice for you. It essentially counts as a free hard check to M-Meta and Lando-T, as well as a large variety of Fairies and Dragons with sets that don't pack Fire coverage (ex. non-Firium Z Garchomp). The set is pretty self-explanatory, with Gyro Ball being its best STAB, EQ for M-Meta, Heatran and friends, and HP Ice for Lando-T, Garchomp, Salamence, and the odd Dragonite. Protect is also an option over HP Ice, but I'd only recommend this if you're running some hard-dog stall, or have some reliable manner of residual damage on your team elsewhere that you want to rack up alongside Leftovers, which is your only source of recovery. That, and you are practically forced to run Moonblast on Fini over any other option to guarantee some decent damage on Dragon-types.
Tapu Fini catches a good variety of threats that Bronzong can't handle, such as Char-X, Ash-Greninja, Weavile, Heatran, Hydreigon, the whole nine yards. It also provides a fast Taunt, which is very welcome for a partner that is pretty vulnerable to Taunt itself. Scald is slashed first more due to the fact that Bronzong handles Dragons anyways (even the ones with SSSS), but in case you prefer doubling down, then Moonblast is your best bet. You can run Nature's Madness + Brine as well, if you're a fucking savage.
This core loves teammates that can put out residual damage, whether it's through Spikes, status effects, Rocky Helmet, Leech Seed, or some varied combination of them all. Things like AV Alolan Muk are great at that thanks to Poison Touch, while also being able to tank some serious punishment from the likes of Volcarona, Charizard-Y, and Bloom Doom Heatran.
Incidentally, Protean Greninja packing Gunk Shot, Kyurem-B, Hoopa-U, and a well-played Mold Breaker Excadrill kinda tear this core to shreds, to say nothing of the other fifty goddamn wallbreakers in the tier. Knock Off is particularly deadly, since you have no reliable recovery in this core.
Scarf Nihilego is an interesting partner, since it can outspeed things like +1 Volcarona and Char-X, then OHKO with Power Gem. To boot, it also benefits from the fact that Bronzong sponges Psychic and Ground attacks like no tomorrow. One of the best speed control options this core can be teamed up with.
A-Muk is a more defensive approach to roughly the same outcome, but A-Muk also happens to be one of the very few viable Ghost + Dark resists in the tier. A-Marowak, eat your heart out. Between Pursuit and Rock Slide, an AV set can dispatch a good number of the same things that Scarf Nihilego can, while also being fuck-off levels of tanky at the same time. Do be wary of the fact that neither A-Muk nor the core itself has any reliable recovery, and A-Muk is still really slow. Enough offensive pressure will topple this core in no time, unless you bring countermeasures.
Speedy U-turners help you make key offensive and defensive plays, as well as bring in any threatening sweeper you may have. Phero, Koko, and Ash-Gren are good examples of this, though some care needs to be taken to not make your team just a typical offense cut-out with Bronzong + Fini pasted in. Any of these threats can scare away Dark-types that carry Poison coverage by being faster and threatening with speedy, super-effective U-turns.
Threatening sweepers are a dime a dozen in the meta, but some pertinent examples are Volcarona and Zard-X since they appreciate the Defog support from Fini, SSSS Lando-T and Zygarde due to this core's resilience to opposing Fairies and bulky waters (especially if you opt for Toxic on SubCoil Zygarde), and SSSS Gyarados because it drops its pants and loudly evacuates its bowels all over any unprepared opponent.