Trainers who captured Vibrava in the Grand Underground will have the ‘mon at Levels 58–63. As a result, Flygon can be evolved with just one Rare Candy.
Those that captured Trapinch via the PokeRadar method on Route 228 will need to raise its levels by 20 as it evolves into its final form at level 45.
After completing the National Pokedex, the simplest way to obtain Flygon is to visit the Pokemon League. When you arrive, utilise your Explorer Kit to gain entrance to the Grand Underground.
The Big Bluff Cavern Hideaway is on the right. Vibrava spawns frequently in this room and has a high level.