Submitting Blissey as a Counter
Blissey (F) @ Heavy-Duty Boots Ability: Natural Cure EVs: 252 HP / 252 Def / 4 SpD Bold Nature - Seismic Toss - Toxic - Soft-Boiled - Teleport
Blissey, although its usage has been falling, is still the tiers best specially defensive pivot, and being so bulky, can shrug off any of Victini's attacks with ease. The Victini user at best has to hope for a crit/spdef drop from focus blast to actually break through bliss, as it slowly gets poisoned to death.
+1 252 SpA Victini Focus Blast vs. 252 HP / 4 SpD Blissey: 252-298 (35.2 - 41.7%) -- 83.4% chance to 3HKO after Leftovers recovery
As long as Blissey is above 40, it can continously spam Softboil and outheal the Victin until it gets a chance to Toxic/SToss and slowly whittle it down.