This Secret Mission is reached by walking out of the door behind Dante at the start of Mission 11.
A Blue Orb Fragment is located high in the air above a pack of Sin Scissors, with the goal being to use Enemy Step to get high enough to grab it. Having Air Hike makes this significantly easier.
The only really tricky part of this mission is figuring out precisely where the Blue Orb Fragment is: it is in the middle of the path, about three-quarters of the way from the doorway to the large weed on the right-hand side. Once properly positioned, keep jumping straight up until a Sin Scissors is in the right place for the Enemy Step.