Instructions
Complete the game by beating the Elite Four and the final champion. Get the ferry ticket from your character's father and use it to travel to the island where the Battle Tower is located.
Level up your Pokémon up to level 50 at least to compete in the Battle Tower. Your Pokémon needs to be at least level 100 in order to compete in the second battle room of the tower.
Take your best three Pokémon with you into the tower. Select your strongest Pokémon that are resistant to the most attacks. You will not be able to change your Pokémon during the challenge so it is vital to have a good selection from the start. Sites like SEREBII (see Resources) have lists of the Pokémon in the tower, so use these as a guide when choosing your own party.
Give each of the Pokémon you are taking into the tower an item to hold. Each item must be unique, as you are not allowed to take two of the same items into the tower. Items like "Quick Claw" are useful as it will increase your chance of attacking first while something like "Leftovers" will gradually increase the Pokémon's health during battles. Normal items cannot be used in the battle tower and Pokémon will be healed automatically after each battle.
Adapt your strategies and keep trying. Getting knocked out in the Battle Tower will reset your progress, so save between battles and learn from your mistakes. Reload your latest saved game after each defeat and change your tactics. Only by winning 100 battles in a row will you have fully beaten the Battle Tower. You cannot use any of the Legendary Pokémon in the tower. so focus on levelling up your normal Pokémon.