Instructions
Walk up to the member of the Elite Four that you are currently supposed to fight. Talk to the person in the middle of the room to start the Pokemon battle.
Lose the fight on purpose. Use ineffective moves whenever possible to give your opponent the upper hand. Use techniques that won't harm the enemy Pokemon as much, such as Poison attacks against Koga's Muk.
Save the game when you are transported out of the Indigo Plateau. This ensures that you won't be forced to replay the Elite Four until you are ready to do so.