Hardest NPC to ever exist in main series Pokemon

Hi . What are your opinions about hardest NPC (other than champion) to battle in main series Pokemon Games .

Rules

  1. NPC must not be Champion
  2. No Gym Leader / Elite 4
  3. No Gamefreak People
  4. No Rivals
  5. No Evils
    What are your opinions

Since I haven’t Played any Pokemon 3ds , switch games . I think the hardest Battle is with

Area 10 White Tree Hollow / Black Skyscraper Boses of White 2 / Black 2