Clefable @ Sticky Barb
Ability: Magic Guard
EVs: 252 HP / 252 Def / 4 Spe
Bold Nature
IVs: 0 Atk
- Moonblast
- Stealth Rock
- Trick
- Soft-Boiled
As mentioned by my peers above, the Toxapex/Amoonguss + Mandibuzz cores are the most popular and effective way to prevent Urshifu from breaking. The reason that these cores are so effective are due to the fact that Regenerator allows a Toxapex or Amoonguss to switch in on a Wicked Blow, without netting a large amount of damage, and then pivot into Mandibuzz afterwards. This sequence allows a Toxapex/Amoonguss + Mandibuzz core to check Choice-Band Urshifu's Dark, Fighting, and Poison coverage.
Sticky Barb Clefable is an amazing partner with Urshifu, since it is able to lure in an opposing Toxapex or Amoonguss fairly easily; an opposing Toxapex wants to Knock Off Clefable, while an opposing Amoonguss wants to Sludge Bomb, Spore, or Stun Spore Clefable. Using Trick to give the opposing Toxapex or Amoonguss a Sticky Barb not only removes a potential Black Sludge or Rocky Helmet that was preventing Urshifu from powering through them with Wicked Blow, but causes these Pokemon to lose an eighth of their health each turn. Stealth Rock furthers the passive damage that Toxapex or Amoonguss recieves, upon switching in, to 25%. Even without any prior damage, an Adamant Choice-Band Urshifu can deal 75% to a 64+ defensive Toxapex with a max roll, and can deal 75% to a 220+ defensive Amoonguss with a high roll:
252+ Atk Choice Band Urshifu Wicked Blow vs. 252 HP / 64+ Def Toxapex on a critical hit: 195-231 (64.1 - 75.9%)
252+ Atk Choice Band Urshifu Wicked Blow vs. 252 HP / 220+ Def Amoonguss on a critical hit: 301-355 (69.6 - 82.1%)