This answer is entirely Theoretical
Hacked Answer: Theoretically, in a double battle, a Pokémon with Simple, could use Extreme Evoboost (as a move, without a Z-Crystal), raising it's Attack, Defense, Special Attack, Special Defence, and Speed by four stages, the second Pokemon could out speed the rest and use Instruct on it, one of the opponent's Pokémon could use Instruct on it (maxing out all it's main stats), then FINALLY, the other Opponent's Pokémon uses Acupressure (raising either Accuracy or Evasion). (credit to KRLW890 for the Instruct & Acupressure strategy)
Legal Answer: (still theoretically) In a double battle, an Eevee could use Extreme Evoboost, raising it's Attack, Defense, Special Attack, Special Defense, and Speed by two stages, the second Pokemon could use (almost any Pokemon) Swagger on it, one of the opponent's Pokémon could use Swagger on it (maxing out it's attack), then FINALLY, the other Opponent's Alcremie uses Docorate. In total, you'd have raised Attack by six stages, Defense by two stages, Special Attack by four stages, Special Defense by two stages, and Speed by two stages.