My initial opinion on Victini is leaning towards broken, although it is still pretty early in the suspect test to tell if it is. Offence seems to have a much smaller problem with this mon owing the the fact that there would be quite a few reliable enough checks to it such as Maero / Gatr / Krook. How ever the same can't be said for balance or bulkier teams sadly. Outside of being able to check it switching in is an immense struggle. Being a fan of playing balance I hate switching into this monster mainly because scouting for its set is a pain. In my opinion the Choice Band set while powerful, seems outclassed by the unpredictability of the expert belt set which is what makes finding a switch in rather problematic. Tini's amazing movepool allows it to beat some of the common switch ins to it such as Glaciate for Dragons or Energy Ball / Thunder / Grass Knot for bulky waters. Even having access to Focus Blast allows it to do a solid chunk to things like Snorlax while OHKO'ing some of the checks to BandTini like Hydrei ( provided rocks are up). While this may be a very scary mon when I was testing I found two rather decent answers to it. One being Defensive Mega Ampharos, and the other being mixed def Arcanine w/ Crunch. But these face trouble with Life Orb Mixed sets sadly. While they do take under 50% they aren't particularly hard to wear down (which is why i ended up running both on the same team along with swampert) . Personally I would be happy to see the back of Victini.
Mega Amphy Calcs
Band
252 Atk Choice Band Victini V-create vs. 248 HP / 252+ Def Mega Ampharos: 127-150 (33.1 - 39.1%) -- 100% chance to 3HKO
252 Atk Choice Band Victini Zen Headbutt vs. 248 HP / 252+ Def Mega Ampharos: 114-135 (29.7 - 35.2%) -- 18.1% chance to 3HKO
E-Belt
156 SpA Victini Psychic vs. 248 HP / 0 SpD Mega Ampharos: 105-124 (27.4 - 32.3%) -- guaranteed 4HKO
156 SpA Expert Belt Victini Glaciate vs. 248 HP / 0 SpD Mega Ampharos: 122-144 (31.8 - 37.5%) -- 88.5% chance to 3HKO
156 SpA Expert Belt Victini Dazzling Gleam vs. 248 HP / 0 SpD Mega Ampharos: 149-178 (38.9 - 46.4%) -- guaranteed 3HKO
156 SpA Victini Focus Blast vs. 248 HP / 0 SpD Mega Ampharos: 93-110 (24.2 - 28.7%) -- 98.8% chance to 4HKO
(From these it's rather easy to see how much of a pain Life Orb is)
Arcanine Calcs
Band
-1 252 Atk Choice Band Victini Bolt Strike vs. 248 HP / 144 Def Arcanine: 120-142 (31.3 - 37%) -- guaranteed 4HKO after Leftovers recovery
-1 252 Atk Choice Band Victini Zen Headbutt vs. 248 HP / 144 Def Arcanine: 111-132 (28.9 - 34.4%) -- 99.8% chance to 4HKO after Leftovers recovery
E-Belt
156 SpA Victini Thunder vs. 248 HP / 112+ SpD Arcanine: 89-105 (23.2 - 27.4%) -- possible 5HKO after Leftovers recovery
156 SpA Victini Psychic vs. 248 HP / 112+ SpD Arcanine: 109-129 (28.4 - 33.6%) -- 96.2% chance to 4HKO after Leftovers recovery
156 SpA Victini Focus Blast vs. 248 HP / 112+ SpD Arcanine: 96-114 (25 - 29.7%) -- 0% chance to 4HKO after Leftovers recovery
Edit: After a little more testing, realised that Mega Camerupt does a decent job of beating Victini. credits to
Yifeng senpai for the idea
Camel Calcs
Band
252 Atk Choice Band Victini V-create vs. 248 HP / 160+ Def Mega Camerupt: 143-168 (41.6 - 48.9%) -- guaranteed 3HKO
252 Atk Choice Band Victini Zen Headbutt vs. 248 HP / 160+ Def Mega Camerupt: 127-151 (37 - 44%) -- guaranteed 3HKO
E-Belt
156 SpA Victini Psychic vs. 248 HP / 100 SpD Mega Camerupt: 99-117 (28.8 - 34.1%) -- 1.5% chance to 3HKO
156 SpA Expert Belt Victini Hidden Power Water vs. 248 HP / 100 SpD Mega Camerupt: 216-254 (62.9 - 74%) -- guaranteed 2HKO
156 SpA Victini Focus Blast vs. 248 HP / 100 SpD Mega Camerupt: 88-104 (25.6 - 30.3%) -- guaranteed 4HKO