Instructions
Obtain all eight Johto badges and fly to New Bark Town. Talk to Professor Elm to obtain the Master Ball.
Fly to Ecruteak City and heal your team at the Pokemon Center. Enter the Kimono House to the north and defeat the Kimono Sisters in battle to obtain the Tide Bell.
Fly to Cianwood City and surf east until you see four islands arranged in a square formation. Use Whirlpool to cancel out the swirling water and enter the northeastern island.
Use Flash to light up the cave. Jump down the ledges until you are given a choice between two ledges. Take the upper ledge and go down the ladder.
Follow the path and head down the next ladder. Head south and talk to the monk. He will move out of your way.
Continue south down the slope and enter the cave to find Lugia.