Apologies for the late submission. :/ Hopefully, I can still submit this, but if not, that's okay.
Sets/The Core:
Skarmory
EVs: Defense: 252 / HP: 252 / Special Defense: 4
Nature: Impish (+Def, -SpA)
Nature:
Iron Defense
Toxic
Defog
Roost
Ho-Oh
EVs: Special Defense: 200 / Defense: 56 / HP: 252
Nature: Careful (+SpD, -SpA)
Sacred Fire
Toxic
Roost
Whirlwind
Calcs and Brief, Ensuing Explanations:
Calcs for Ho-Oh:
+2 252+ SpA Xerneas Psyshock vs. 252 HP / 56 Def Ho-Oh: 198-233 (47.5 - 56%) -- 25.8% chance to 2HKO after Leftovers recovery ---> This would be the risk most would cite of running a heavily SpD boosted Ho-Oh, and it is a reasonable concern. However, this calc shows how the slight investment in physical Defense in Ho-Oh makes an important distinction in checking the Xern set.
+2 252+ SpA Fairy Aura Xerneas Moonblast vs. 252 HP / 200+ SpD Ho-Oh: 124-147 (29.8 - 35.3%) -- guaranteed 4HKO after Leftovers recovery
240+ SpA Life Orb Dark Aura Yveltal Dark Pulse vs. 252 HP / 200+ SpD Ho-Oh: 134-160 (32.2 - 38.4%) -- 1.6% chance to 3HKO after Leftovers recovery
240+ SpA Life Orb Yveltal Oblivion Wing vs. 252 HP / 200+ SpD Ho-Oh: 101-121 (24.2 - 29%) -- possible 5HKO after Leftovers recovery
Calcs for Skarmory:
+2 252 Atk Rayquaza-Mega Dragon Ascent vs. 252 HP / 252+ Def Skarmory: 142-168 (42.5 - 50.2%) -- guaranteed 3HKO after Leftovers recovery
+2 252 Atk Rayquaza-Mega Dragon Ascent vs. +2 252 HP / 252+ Def Skarmory: 72-84 (21.5 - 25.1%) -- possible 5HKO after Leftovers recovery
252 Atk Rayquaza-Mega Dragon Ascent vs. +2 252 HP / 252+ Def Skarmory: 36-42 (10.7 - 12.5%) -- possibly the worst move ever ---> I would like to highlight here how it is the Damage Calculator that added the "possibly the worst move ever" annotation, not me. XD That goes to show how significant the defense of Skarm is here, particularly against a non-V-Create Mray set.
248 Atk Necrozma-Ultra Outrage vs. 252 HP / 252+ Def Skarmory: 67-79 (20 - 23.6%) -- possible 6HKO after Leftovers recovery
+2 248 Atk Necrozma-Ultra Outrage vs. 252 HP / 252+ Def Skarmory: 134-158 (40.1 - 47.3%) -- guaranteed 3HKO after Leftovers recovery
248 Atk Necrozma-Ultra Outrage vs. +2 252 HP / 252+ Def Skarmory: 33-40 (9.8 - 11.9%) -- possibly the worst move ever ---> Again, not my annotation. lol
252+ Atk Arceus Giga Impact vs. 252 HP / 252+ Def Skarmory: 72-85 (21.5 - 25.4%) -- 0.4% chance to 4HKO
+2 252+ Atk Arceus Giga Impact vs. 252 HP / 252+ Def Skarmory: 144-170 (43.1 - 50.8%) -- 3.9% chance to 2HKO
252+ Atk Arceus Giga Impact vs. +2 252 HP / 252+ Def Skarmory: 36-43 (10.7 - 12.8%) -- possible 8HKO
252+ Atk Arceus Giga Impact vs. 252 HP / 252+ Def Skarmory: 72-85 (21.5 - 25.4%) -- 0.4% chance to 4HKO
More Detailed Analysis of the Core:
For some time now, I have used a SpD-invested Ho-Oh set to great effect actually. Though it can't check Mray at that point, as can be seen above it functions excellently as a Xern check. And frankly, it works well against other Special Attack powerhouses as well. As can be determined by the calcs above, Ho-Oh is formidable here against even LO SpA-boosted Yveltal: Even with the additional boost from Dark Aura, Dark Pulse does fairly marginal damage, which could be dangerous considering the impact of Sacred Fire and Toxic and its lack of recovery moves.
In turn, Skarmory's physical Defense is quite impressive and is responsible for countering much of the team. Even significantly powerful physical attackers like Ultra, Mray, and Ekiller can't cause significant damage without sustaining significant damage from Toxic.
Overall, I believe this core of Pokémon is quite formidable against the team pichus provided. Even against Shuckle there is Skarmory with Defog and Toxic to wear it down and not in turn be inflicted with Toxic. Let me know what you think, if you'd like!