How Do I Get Rayquaza for Pokemon Sapphire on GBA?

Defeat the Elite Four in the Indigo Plateau.

Fly to Pacifidlog Town and approach the eastern coast. Press "Start" to bring up the in-game menu, and select "Pokemon." Choose a Pokemon with the Surf ability to surf onto the water.

Surf east onto Route 131. Hug the northern wall as you continue surfing east. You will come across a small hole in the wall.

Surf upward through the hole in the wall until you see a cave opening. Approach the cave and walk through the entrance.

Head north through the small cave, and enter the Sky Pillar.

Walk up the stairs to the second floor of the Sky Pillar. Press "Start" and open your Key Items bag. Select the "Mach Bike."

Ride over the cracks on the floor with your Mach Bike. Head up the stairs to the third floor.

Ride up the staircase on the right with your bike to reach the fourth floor. Quickly ride over the cracks on the floor until you reach the middle. Pause to let the floor crack below you, dropping you down to the previous level.

Go up the staircase to reach the other side of the fourth floor. Head up the left staircase to the fifth floor.

Continue up the last staircase to reach the rooftop. Approach Rayquaza in the middle, and press "A" to challenge it to a battle.

Battle Rayquaza and weaken it until its hit points are low and in the red zone. Attack it with moves such as Thunder Wave and Hypnosis to afflict it with Paralyze and Sleep status ailments.

Throw an Ultra Ball at Rayquaza to catch it. If it breaks free, continue throwing Ultra Balls or other strong Poke Balls at it to capture it.