Instructions
Travel either to Route 114 or Route 115. Both routes have entrances to Meteor Falls. Enter Meteor Falls from either route.
Make your way to the second basement floor by taking the ladder down from the first floor and then using a Pokemon with "Surf" to swim across the body of water on the first basement floor. Then go down the ladder across the water to enter the second basement floor.
Walk around the second basement floor until you are attacked by a Bagon. Use Ice-type or Dragon-type Pokemon to weaken the Bagon.
Use a Pokeball to capture the Bagon once its health is flashing red.