Alolan Ninetales is pretty standard if you want to use Aurora Veil, but since that's easy to find. Here's an offensive Atails set that I think is pretty much optimised. Replace Icy Rock with Leftovers if not paired with Arctozolt (not covering here unless requested since the sets are pretty standard). Also this is for singles, so make sure you use a different set for BSD / VGC.
Ninetales-Alola @ Icy Rock / Leftovers
Ability: Snow Warning
EVs: 40 HP / 32 Def / 228 SpA / 208 Spe
Timid Nature
IVs: 0 Atk
- Nasty Plot
- Psyshock
- Freeze-Dry
- Blizzard
Nasty Plot is needed for this set. Because of Ninetales's poor Special Attack and Defense, Calm Mind is not an option. Blizzard is also needed for Rillaboom, as Freeze Dry will not OHKO without a boost. Psyshock OHKOs Nihilego at +1, and can threaten Blissey at +2, with +4 threatening a 2HKO under hail (though Arctozolt is better equipped for Blissey). Freeze Dry 2HKOs Slowbro and threatens an OHKO at +2, and is important to stop it from walling Ninetales completely. Hidden Power Ground is an option over Psyshock in National Dex on Pokémon Showdown, however the resulting decrease in damage presented by Blizzard along with the length of Hail can be problematic, and both Heatran and Magnezone survive one HP Ground, and then OHKO back. Even specially defensive Heatran sets, the more common set, live a +2 HP Ground.
These EVs prevent a Life Orb Rillaboom from OHKOing Ninetales with Grassy Glide, the only way Rillaboom will outspeed Ninetales. 208 speed and a Timid nature allow Ninetales to outspeed a +0 Nihilego and threaten an OHKO with Psyshock. While Moonblast is a nice STAB move to have, the combination of Blizzard and Psyshock threatens anything that Moonblast would regularly, barring Pokémon like Bisharp (who would still live a Moonblast and OHKO back with Iron Head), and so is useless.
Calcs:
252+ Atk Life Orb Rillaboom Grassy Glide vs. 40 HP / 32 Def Ninetales-Alola in Grassy Terrain: 251-296 (84.5 - 99.6%) -- guaranteed 2HKO after Grassy Terrain recovery
+2 228 SpA Ninetales-Alola Psyshock vs. 80 HP / 0 Def Nihilego: 450-530 (118.7 - 139.8%) -- guaranteed OHKO
+2 228 SpA Ninetales-Alola Psyshock vs. 252 HP / 252+ Def Blissey: 225-265 (31.5 - 37.1%) -- guaranteed 3HKO after hail damage
+4 228 SpA Ninetales-Alola Psyshock vs. 252 HP / 252+ Def Blissey: 337-397 (47.1 - 55.6%) -- 78.5% chance to 2HKO
228 SpA Ninetales-Alola Freeze-Dry vs. 252 HP / 4 SpD Slowbro: 198-234 (50.2 - 59.3%) -- guaranteed 2HKO
+2 228 SpA Ninetales-Alola Freeze-Dry vs. 252 HP / 4 SpD Slowbro: 390-462 (98.9 - 117.2%) -- 93.8% chance to OHKO (Guaranteed OHKO after Hail)
228 SpA Ninetales-Alola Hidden Power Ground vs. 0 HP / 0 SpD Magnezone: 204-244 (72.5 - 86.8%) -- guaranteed 2HKO after hail damage
+2 228 SpA Ninetales-Alola Hidden Power Ground vs. 252 HP / 136+ SpD Heatran: 284-336 (73.5 - 87%) -- guaranteed 2HKO after hail damage and Leftovers recovery
EDIT: I do want to add that Choice Specs is sorta viable but loses out on the extended hail turns for Arctozolt to use, unless this is in doubles formats, where longer weather is less desirable (it should be noted that this is prone to weather resetting and so a support set with Hail, Freeze-dry, Blizzard, and Protect is likely more desirable as a whole).