Whoops, I completely forgot about this. haha Thanks for the reminder, Arushii.
Fairyceus
EVs and Moves:
HP: 200
Defense: 252
Special Attack: 52
Nature: Bold (+Def, -Atk)
Judgment
Recover
Calm Mind
Fire Blast
Overview/Analysis: Fairyceus is a versatile Pokémon in the AG meta, and this pairing intends to make use of some of its strengths. Fairyceus is a strong Marshadow counter, which among other reasons is why I've chosen it. While Mega Mewtwo X does indeed outspeed Marshadow (unless it's Scarfed, which would be weird lol), the potential for Marshadow to survive a hit with Focus Sash for example and steal the stat ups with Spectral Thief (which, as a reminder, penetrates through Substitutes) necessitates a better check on it. Thus, there is physically defensive Fairyceus:
--- +1 252 Atk Life Orb Marshadow Spectral Thief vs. 200 HP / 252+ Def Arceus-Fairy: 177-211 (41 - 48.9%) -- guaranteed 3HKO ---
Judgement will also OHKO Marshadow, so given that threat and the little amount of damage Marsh can inflict, this will grant Fairyceus a free move, which could go towards Calm Mind.
I chose to include Fire Blast on Fairyceus for a few reasons. It is not uncommon to run Fire Blast as Steelceus could threaten it otherwise; however, Low Kick from MMX could handle that threat, so there must be more. Fire Blast here can also be used effectively towards Dusk-Mane and Celesteela. The perk of this option is even if Necrozma Ultra-Bursts, Judgment will deal with it quickly without incurring fatal damage. Here are some calculations that testify to the strength of Fairyceus here:
252 Atk Necrozma-Dusk-Mane Sunsteel Strike vs. 200 HP / 252+ Def Arceus-Fairy: 240-284 (55.6 - 65.8%) -- guaranteed 2HKO (Indeed powerful, but not a OHKO)
52 SpA Arceus-Fairy Fire Blast vs. 0 HP / 4 SpD Prism Armor Necrozma-Dusk-Mane: 135-159 (40.2 - 47.4%) -- guaranteed 3HKO --> which turns into...
+1 52 SpA Arceus-Fairy Fire Blast vs. 0 HP / 4 SpD Prism Armor Necrozma-Dusk-Mane: 201-237 (60 - 70.7%) -- guaranteed 2HKO
Overall, Fairyceus can be a powerful partner to MMX. With Mewtwo's speed, physical attack, and Substitute, it is a weapon on its own, and with the bulk and variety of Fairyceus to back it up, this could be a formidable team.