Nominating a different variation of Reuniclus as a
counter.

Reuniclus @ Assault Vest
Modest Nature
Ability: Regenerator
EVs: 248 HP / 116 SpA / 140 SpD / 4 Spe
- Psyshock
- Future Sight
- Focus Blast
- Knock Off
With its extremely high HP and decent bulk, Reuniclus is well-placed to take advantage of Regenerator. With Future Sight, it can force predictable switches on the opponent's end. With an Assault Vest, it can easily set up this Future Sight in front of almost any special attacker. And with Focus Blast and Knock Off, it has the tools to punish those switches by itself.
Against Kyurem, Reuniclus can easily take an attack, and fire off a Future Sight. Then, it is just a matter of using Psyshock to constantly pressure Kyurem's substitutes and make way for Future Sight to hit. This pressure also means that Kyurem can only ever have a Substitute up or be at full health, not both. From maximum health, this combo deals a
minimum of 85.1% damage, in most cases allowing for a KO with Focus Blast as the Kyurem is forced to Roost to recover its health. (You have exactly enough Psyshock PP to attempt this 3 times.)
Calcs:
200 SpA Kyurem Freeze-Dry vs. 248 HP / 140 SpD Assault Vest Reuniclus: 73-87 (17.2 - 20.5%) -- possible 5HKO
116+ SpA Reuniclus Psyshock vs. 56 HP / 0 Def Kyurem: 138-163 (34 - 40.2%) -- 39.2% chance to 3HKO after Leftovers recovery
116+ SpA Reuniclus Future Sight vs. 56 HP / 0 SpD Kyurem: 207-244 (51.1 - 60.2%) -- 89.1% chance to 2HKO after Leftovers recovery
116+ SpA Reuniclus Focus Blast vs. 56 HP / 0 SpD Kyurem: 276-326 (68.1 - 80.4%) -- guaranteed 2HKO after Leftovers recovery