This secret area is located in the first stage of the Airship Core mission.
Near the end of the stage, you will be able to see a couple lasers guarding an area. I was able to get through them by bumping into them, receiving damage, becoming invulnerable from the damage, and running through the lasers.
Complete the stage, open the chest, and leave to unlock the achievement.
Tips for completing the level:
* You can shoot the spiky laser balls to trigger them to open up.
* Jumping and shooting the ground will propel you upwards a short distance.
Video guide: