Chopin Alkaninoff said:
Corsola (Kyogre) @ Assault Vest
Ability: Regenerator
EVs: 252 HP / 4 SpA / 252 SpD
Calm Nature
- Scald
- Ice Beam
- Earth Power
- Power Gem
You need a Primsea Palkia stopper? Or Desolate Land Reshiram? This does the job. Yes, it inherits from Corsola, because it gives Earth Power that 2HKO 252 HP/0 SpD Primal Groudon, which means it's has to be careful to switch in. It can pretty much tank almost any Special Attacks and hit back pretty hard with its 150 SpA (unlike Goodra who doesn't have power to be threatening). Inheriting from Slowbro is also an option because it gives you something like Focus Blast or Flamethrower, though you need 252 SpA for Focus Blast to even have chance to 2HKO 252 HP/0 SpD Primal Groudon.
Worth commentary is that Ho-Oh is another Earth Power Regenerator. It won't provide a Water STAB, but it has potential regardless. You could also use Giratina instead if you're inheriting from Ho-Oh -with the advantage of being resistant to Thunder and Solar Beam where Kyogre is vulnerable to both- and Ho-Oh provides STAB Shadow Ball at that point. (No Dragon STAB, but that's okay) Plus U-Turn, which is great for a Regenerator Assault Vest 'mon. Honestly, Giratina seems the superior Regenerator-based Primal Sea/Desolate Land stopper.
Lucario (Slaking) @ Life Orb
Ability: Adaptability
EVs: 252 Atk / 4 Def / 252 Spe
Adamant Nature
- Swords Dance
- Extremespeed
- Close Combat
- Crunch
Okay. Adaptability Slaking with its insanely strong Extremespeed. This is like, the superior Extremekiller that make Normal Arceus looks bad. Of course it's gonna be a threat. A Banded all attacks set hits harder even though it can't use Swords Dance, so it can be a better choice than Life Orb.
Regigigas has somewhat worse Physical bulk but overall better Special bulk, so it's an alternative. Slaking is the superior choice if you want to go mixed for some reason, though, albeit only by a little bit.
---
Some ideas that come to mind to me:
Palkia inheriting from Kingdra and running the Critdra set. Unimpressed by Unaware (And for that matter Special Defense boosting) and packing by monstrous,
spammable firepower. There's only one Ubers-tier Fairy, too, and...
252 SpA Palkia Scald vs. 248 HP / 252+ SpD Xerneas on a critical hit: 160-189 (35.1 - 41.5%) -- guaranteed 3HKO
... well, if Xerneas
isn't bulky it can't actually tank Critpalkia, particularly since it risks Burns from the Scald. Also? Flash Cannon is in Kingdra's movepool.
252 SpA Palkia Flash Cannon vs. 248 HP / 252+ SpD Xerneas on a critical hit: 214-252 (47 - 55.3%) -- 69.5% chance to 2HKO
Yikes.
Alternatively you could take Dialga for it.
252 SpA Dialga Flash Cannon vs. 248 HP / 252+ SpD Xerneas on a critical hit: 320-378 (70.3 - 83%) -- guaranteed 2HKO
No surprise that Dialga has no fear of the one Fairy in the tier. The one thing that can wall it is Aegislash, which takes a mere
252 SpA Dialga Draco Meteor vs. 252 HP / 252+ SpD Aegislash-Shield on a critical hit: 96-113 (29.6 - 34.8%) -- 10.3% chance to 3HKO
... yeah, usually won't even 3HKO, no need for Leftovers. Alas, Kingdra provides no alternatives for beating it -Scald is actually weaker than resisted Draco Meteor.
Adaptability Extreme Speed? How about Belly Drum Extreme Speed? Just inherit from Linoone.
Yveltal inheriting from Raticate could be strong. Guts+Swords Dance Sucker Punch is nasty against offensive Pokemon not leaning on Extreme Speed, and most teams will only have one or two Extreme Speeders anyway.
A few Ubers can pull off Geomancy builds. Zekrom, Mewtwo, Lugia, Dialga, and Genesect I guess all get STABs and good enough stats to pull their weight. Mewtwo and Lugia are probably the overall best alternatives to Xerneas itself, thanks to STAB Psyshock, though you'll have to watch out for Sucker Punch Yveltal -regular Dark Aura Yveltal is an easy OHKO even on max HP Mewtwo with Sucker Punch. (Max HP Lugia will usually survive a Life Orb and Dark Aura backed Sucker Punch. Usually) Though...
252+ SpA Fairy Aura Mewtwo Moonblast vs. 0 HP / 0 SpD Yveltal: 348-410 (88.5 - 104.3%) -- 25% chance to OHKO
... Yveltal can't just freely switch in.
Naturally, Primal Groudon presents something of an issue to Geomancy builds, too, as they can't pick on its one weakness and it's powerful and omnipresent. Still.
+2 252+ SpA Mewtwo Psyshock vs. 252 HP / 252+ Def Primal Groudon: 168-198 (41.5 - 49%) -- guaranteed 3HKO
+2 252+ SpA Mewtwo Focus Blast vs. 252 HP / 0 SpD Primal Groudon: 356-419 (88.1 - 103.7%) -- 25% chance to OHKO
They can push past it if Primal Groudon is scouted (and weakened, if a more defensive build) a little before heading out.
Here's an odd thought: Tail Glow Giratina. (Origin? Probably Origin if inheriting from Manaphy -even with all the Primal Sea, counting on Hydration is iffy-, but Volbeat gives Prankster Tail Glow and Prankster Roost) Both Manaphy and Volbeat provide Shadow Ball, and as multiple of Ubers premiere walls are weak to Ghost (Aegislash, Giratina, Lugia...) STAB Tail Glow Shadow Ball is quite nasty, even with Chansey being a thing. (Note that Volbeat's access to Encore can be used to ruin non-Magic Bounce Chansey attempts to mess with you) Manaphy has the overall superior movepool of course, but Volbeat has key advantages, including Giga Drain potentially extending longevity.
Also note that Manaphy has Psychic, so Tail Glow Mega Mewtwo Y is totally viable.
+3 252+ SpA Mega Mewtwo Y Psychic vs. 4 HP / 252 SpD Eviolite Chansey: 280-330 (43.6 - 51.4%) -- 6.6% chance to 2HKO
Not as good as Nasty Plot/Simple Calm Mind Psyshock at beating Chansey, obviously, but holy crap anyway. Just beware Yveltal.