Instructions
Defeat the Elite Four and the Pokemon League Champion, and use a Pokemon's "Fly" move to get to Snowpoint City in the far north of the Sinnoh region.
Catch the ferry from Snowpoint City to the Fight Area on the island to the northeast.
Go to the Battle Tower in the Fight Area, and go through the middle door to start Standard Battles.
Win 20 consecutive battles -- each battle will be harder than the last.
Face Tower Tycoon Palmer in the 21st battle. He has a Dragonite, a Milotic and a Rhyperior, all fixed at Level 50. Use an Ice-type against Dragonite. Use a Grass or Electric-type against the Water-type Milotic, and a Water or Grass-type against Rhyperior, which is a dual Rock/Ground-type.
Win another 28 Standard Battles in a row.
Face Tower Tycoon Palmer for the second time as your 49th consecutive battle. This time he has three Legendary Pokemon -- Heatran, Regigigas, and Cresselia, again all at Level 50. Heatran is a Fire/Steel-type, which makes it very weak to Ground-type Pokemon, while the Normal-type Regigigas is only vulnerable to Fighting-type attacks. Psychic-type Cresseila is weak against Bug, Ghost and Dark-type Pokemon.