Instructions
Go to the Battle Frontier area to the West of Olivine City in the Johto region.
Enter the Battle Tower in the north of the Battle Frontier and go through the middle door of the three inside.
Win 20 battles in a row and you will face Tower Tycoon Palmer for the first time.
Use an Ice-type Pokemon to defeat his Dragon/Flying-type Dragonite. His Water-type Milotic is weak against Electric- and Grass-types. Use a Grass- or Water-type to take down his Ground/Rock-type Rhyperior.
Win 28 more consecutive battles, and you can face Palmer for a second time -- and this time all of his team are Legendary Pokemon.
Use a Ground- or Water-type Pokemon to beat his Fire/Steel-type Heatran. A Fighting-type will be strong against his Normal-type Regigigas. Use a Bug-, Dark- or Ghost-type Pokemon to defeat his final Pokemon, the Psychic-type Legendary Pokemon Cresselia.