1- Solo Method
You'll want a bunch of flight upgrades and the upgrade that allows you to take less damage from fire if you want to do this efficiently.
In order to light yourself on fire mid-flight and remain in flight you will need to find a source lava that you can fly through. There are floating platforms above the city, and some of them have "lava waterfalls" flowing from them. Flying through these will light your character on fire, and you can continue to fly in order to build your distance for this achievement/trophy. You can also fly very low and close to an open pool of lava and fly through small "explosions" of lava on the surface which will also work!2 - Co-op Method
This method is a little involved but also works. You will need two people the Ulcier's Edge (Seven Deadly Sins Meele Weapon), which is acquired by completing all 5 marshalling grounds, and then visiting the middle of map using the teleport. The main attack of this weapon will shoot a fireball.
You can light each other on fire (even with friendly damage off) by having one person hover while the other person strikes them with the fireball. Once the hovering person is on fire, they can fly around and then repeat the process until the achievement/trophy unlocks.