Instructions
Go to the Lake of Rage. The lake is located on the northern end of Route 43 and is easily reached by walking due north from Mahogany Town.
Face the lake and have a Pokemon use the move "Surf." This technique allows players to move across the surface of waterways.
Swim to the center of the lake where the massive Red Gyarados is sticking its head out of the water. Save your game before provoking Gyarados, in case you accidentally knock it out.
Face the Red Gyarados and press "A" to start the Pokemon battle. Hit the Level 30 Red Gyarados with Rock- or Electric-type attacks to quickly lower its health bar. Use Ultra Balls, Heavy Balls and Net Balls to capture the Red Gyarados.