From the room where the end portal is located, there's a pathway on the left which leads to an extra area. Here you'll find a flying powerup which requires ten spirits to activate.
For the challenge, you'll need to flame six lamps which are located around the edges of the open area. As long as you're flying straight from lamp to lamp, completing the challenge in under 15 seconds shouldn't be a problem.
Here's a video showing the location and the challenge.
You can also get this achievement with the similar challenge in the cave, but you can only access it once you've learned to climb later in the game.