NOTE: This is not a troll like Smashpass Dialga was.
Mandibuzz @ Leftovers
Ability: Overcoat
EVs: 248 HP / 8 Def / 252 SDef
Calm Nature (+SDef, -Atk)
- Taunt
- Roost
- Whirlwind
- Toxic
Although this may seem similar to Skarmory in terms of movepool, there are several differences; Skarmory has higher physical bulk but with a much lower special bulk. Although Skarmory gets Spikes, we already have Spikes support from Deoxys-S. With heavy investment in SDef, Mandibuzz can effectively wall and kill Ghostceus with Toxic. SubCM sets lack recovery and can be Whirlwinded out with ease, something the pink blobs cannot claim to do.
Against CM Ghostceus:
252SpAtk Spooky Plate +1 Multitype Arceus (Neutral) Focus Blast vs 248HP/252SpDef Leftovers Mandibuzz (+SpDef): 32% - 38% (138 - 163 HP). Guaranteed 4HKO.
252SpAtk Spooky Plate +1 Multitype Arceus (Neutral) Judgment vs 248HP/84SpDef Leftovers Ho-Oh (Neutral): 43% - 51% (180 - 213 HP). Guaranteed 3HKO.
252SpAtk Spooky Plate +1 Multitype Arceus (Neutral) Focus Blast vs 252HP/252SpDef Leftovers Sturdy Skarmory (Neutral): 54% - 64% (183 - 216 HP). Guaranteed 2HKO.
As you can see, even after SR, Mandibuzz can survive 2 +1 Focus Blasts (assuming they both hit) from fully offensive Arceus-Ghost. Ho-Oh, however, stands a chance to get OHKOed, or guaranteed 2HKO unless it spams Roost and did not die on the first turn. Skarmory's negligible SDef means that even when fully invested (standard runs Impish), it is still guaranteed to be 2HKOed after SR, even factoring in Leftovers.
Against SD Ghostceus:
252Atk Spooky Plate +2 Multitype Arceus (+Atk) Brick Break vs 248HP/8Def Leftovers Mandibuzz (Neutral): 38% - 45% (162 - 191 HP). Guaranteed 3HKO.
252Atk Spooky Plate +2 Multitype Arceus (+Atk) Shadow Force vs 248HP/8Def Leftovers Mandibuzz (Neutral): 54% - 64% (231 - 273 HP). Guaranteed 2HKO.
252Atk Spooky Plate +2 Multitype Arceus (+Atk) Shadow Claw vs 248HP/252Def Leftovers Ho-Oh (+Def): 53% - 62% (220 - 259 HP). Guaranteed 2HKO.
Against SD Ghostceus, the most reliable counter is Skarmory, which takes next to nothing from anything the God of Pokemon can throw at it. Mandibuzz comes up next, taking less than 50% damage from Brick Break and being able to Roost it off, while Shadow Force is not only rare, its charge-up turn allows Mandibuzz to Roost freely. Shadow Force also has a meagre 8 PP, allowing Mandibuzz to stall it out before using Toxic. Ho-Oh is OHKOed after SR even with a physically defensive spread.
Overall Mandibuzz is the most reliable counter to Ghostceus, taking on CM variants better than the other two after SR, while being able to take SD versions too.
Overcoat > Big Pecks as only defense dropping move in Ubers is Crunch, by Ttar who summons sandstorm, so I would prefer not having my Leftovers negated by sand
Taunt prevents spikers from setting up on me, whirlwind prevents sweepers from doing the same, roost is for recovery and toxic is to stall pokemon to death
tbh I would rather run skarmory though
As a bonus this pokemon is
innovative, as is Skarmory
Lastly, I want to say that beating Ghostceus is not its only role, its very usable bulk and access to Taunt allow it to fulfil multiple roles. E.g. it can check Darkrai, Darkceus and Deoxys-A as well.