[Anikrahman1995]

Ctrl+F Victini 0 results

Why doesnt victini get a mention in checks/counters? It resists all of Mewtwo's common moves(Shadow Ball is almost never used) and can OHKO with V-Create under sun. Now for damage calculations:
252SpAtk Life Orb Unnerve Mewtwo (Neutral) Psystrike vs 4HP/0Def Victini (Neutral): 35% - 41% (120 - 141 HP). Guaranteed 3HKO.

Mewtwo's strongest attack in neutral weather against Victini is only a 3hko.

Victini is used in sunny weather so,
252SpAtk Life Orb Unnerve Mewtwo (Neutral) Fire Blast in Sun vs 4HP/0SpDef Victini (Neutral): 42% - 49% (144 - 170 HP). Guaranteed 3HKO.
252SpAtk Life Orb +1 Unnerve Mewtwo (Neutral) Fire Blast in Sun vs 4HP/0SpDef Victini (Neutral): 63% - 74% (216 - 254 HP). Guaranteed 2HKO.
-----> This is not a OHKO with Stealth Rock.

Still a 3hko. If Mewtwo uses Calm Mind on the switch, both attacks 2hko but Vitini OHKOs in return. Oh BTW, scarf outspeeds and is also the weakest variant so,

252Atk Victini (Neutral) V-create in Sun vs 4HP/0Def Mewtwo (Neutral): 112% - 133% (400 - 472 HP). Guaranteed OHKO.

BTW, this requires that Stealth Rock not be up on Victini's side of the field as then Psystrike and Fire Blast can 2hko. Hope I could contribute somehow.

EDIT: Why isnt Lugia mentioned in checks(not counters)? It can paralyze Mewtwo, roost on the ice beams to regain multiscale and then fish for parahax before using whirlwind to phaze it out.