So Trick Room Victini is gaining more use, as V-Create is powerful STAB while allowing its speed to keep falling and continued Trick Room sweeping.
I wouldn't recommend Sunny Day, since that wastes a move. Maybe use Ninetale's Drought or just go to Trick Room straight away.
Victini @ Life Orb/ Choice Band
Ability: Victory Star
EVs: 252 HP, 252 Atk, 4 Def
Nature: Brave
- V-create
- Bolt Strike
- Zen Headbutt
- Brick Break/ U-Turn/ Protect
Protect is for doubles; Victini makes a decent doubles Pokemon with Victory Star (and also with Doubles, please remember Life Orb instead of Choice Band).
In singles, Life Orb might be better in some cases, considering that if Victini was Choiced, it would be forced to switch out from Flash Fire Chandelure (which is ever popular in UU). However, Life Orb would sap health & Victini has no recovery method, and to be honest... even un-choiced, I would switch out from Chandelure anyway.
Watch out for Sucker Punch users though. I've also faced Trick Room Victini before, and I just stalled Trick Room out (Slowbro is one Victini's biggest counters, due to resistance to Victini's STAB moves, being a prime Trick Room abuser & having incredible physical bulk).
I would also recommend using a Trick Room set up, and I'll list some examples:
Whimsicott @ Leftovers/ Iron Ball
Ability: Prankster
EVs: 252 HP, 252 Def, 4 Sp. Def
Nature: Relaxed
- Trick Room
- U-Turn
- Encore
- Leech Seed
Whimsicott has the benefit of Prankster, and a switch out move (U-Turn). Iron Ball would work better on Whimsicott than on Victini, since Whimsicott has a high speed of 116, and V-Create would decrease Victini's speed anyway. Plus, Iron Ball isn't a draw back for Whimsicott since it has Prankster - Trick Room and then U-Turn escape. Another upside is that your opponent can't predict this set, because Trick Room Whimsicott is nearly never used.
Porygon2 @ Eviolite
Ability: Trace
EVs: 252 HP, 252 Sp. Atk, 4 Def
Nature: Quiet
- Trick Room
- Recover
- Thunderbolt
- Ice Beam
Vicitni is susceptible to Flash Fire Chandelure. Porygon2's Trace will give it Flash Fire, and Normal typing gives Ghost immunity. Porygon2 also serves as a bulky sweeper as well, and can abuse Trick Room itself, meaning it's not a one trick pony.
Dusknoir @ Leftovers
Ability: Pressure
EVs: 252 HP, 252 Sp. Def, 4 Atk
Nature: Sassy
- Trick Room
- Will-O-Wisp
- Shadow Sneak
- Brick Break/ Pain Split
Dusknoir has the benefit of being immune to Fake Out, and therefore able to set up Trick Room immediately (well, unless Taunted). It's got fantastic bulk, and a wide selection of stalling moves.