>>28414057Assuming no event moves.
>Fire Blast>Focus Blast>U-turn>PsyshockHasty, possibly Mild.
Max Speed, max SpA, remainder HP.
Assault Vest or Choice Scarf, or maybe Choice Specs. Maaaaaybe Wide lens if you are paranoid.
Reasoning: Victory Star adjusts for Fire Blast and Focus Blasts Accuracy. U-turn lets Victini get out of battle easily to preserve Victory Star, and also works well alongside Victini's threatening moves. Psyshock is for hitting specially bulky Pokemon, since Fire Blast will wreck most of everything.
With AV, Victini can tank otherwise quite painful attacks to score a KO on something vital.
With a Scarf, it can either KO something fast and frail before it moves with a powerful Fire Blast, or else Turn out safely against fast threats. Specs lets Victini wallbreak pretty nicely, which works well alongside U-Turn once the opponent realizes how hard Victini is hitting for mindgames.
Wide Lens is generally useless, especially since you are already building Victini with its ability in mind, but if you are absolutely paranoid about missing, I'm pretty sure Fire Blast is effectivly 100% accurate, and Focus Blast is something like 85-90% accurate with a Lens on top of Victory Star.
No matter what build you decide to go with, I advocate heavily for U-turn on it. Victory Star is one of the main reasons to use Victini in the first place, and it can be quite a potent force on any battle. Being able to preserve it effectively and efficiently is a huge boon to Victini, and to the team as a whole.
