Ghost Arceus @ Leftovers
Ability: Multitype
EVs: 252 HP / 160 Def / 90 Spe
Timid Nature
-Recover
-Judgment
-Will-O-Wisp
-Defog (Or some other magical suport move idk)
252 SpA Mega Mewtwo Y Psystrike vs. 252 HP / 160 Def Arceus-Ghost: 166-196 (37.3 - 44.1%) -- 1.2% chance to 2HKO after Stealth Rock
0 SpA Spooky Plate Arceus-Ghost Judgment vs. 0 HP / 4 SpD Mega Mewtwo Y: 258-306 (73 - 86.6%) -- guaranteed 2HKO
252 Atk Mega Mewtwo X Zen Headbutt vs. 252 HP / 160 Def Arceus-Ghost: 130-154 (29.2 - 34.6%) -- 6.3% chance to 3HKO
0 SpA Spooky Plate Arceus-Ghost Judgment vs. 0 HP / 4 SpD Mega Mewtwo X: 302-356 (85.5 - 100.8%) -- 43.8% chance to OHKO after Stealth Rock
252 SpA Life Orb Mewtwo Psystrike vs. 252 HP / 160 Def Arceus-Ghost: 181-214 (40.7 - 48.1%) -- guaranteed 3HKO
0 SpA Spooky Plate Arceus-Ghost Judgment vs. 0 HP / 4 SpD Mewtwo: 330-390 (93.4 - 110.4%) -- guaranteed OHKO after Stealth Rock
GET WREKT KID. Basically, Ghostceus has the virtue of taking almost any hit that Mewtwo and all of its formes have to offer, and of course doing the best against MM2X with the defense investment and WoW. If Mewtwo takes the time to Taunt, it gets 2HKOd. Only normal Life Orb Mewtwo stands a chance of 2HKOing, but MUST hit Ghostceus on the switch with a Psystrike after SR damage in order to win. lel. Did I mention that Ghostceus also doesn't scream "MegaGar bait"? Yep.
0 SpA Spooky Plate Arceus-Ghost Judgment vs. 4 HP / 0 SpD Mega Gengar: 318-374 (121.3 - 142.7%) -- guaranteed OHKO
'Nuff said.