On the following page of our Devil May Cry 5 tutorial, you will find a description of the passage of the second secret mission in the game.
Secret mission 02
Where to find it: Mission 03 - Flying Hunter
Once you destroy the Qliphoth roots, you will uncover the passage to the stairs leading to the recess in the wall. A secret mission is hidden at the bottom. Place Nero accordingly and you will start another challenge.
This time, you have to destroy the Red Empusa before they escape. At first, you won't have any problems, because you'll fight with one empusa. When you defeat it, two more will appear, and at the very end, you will have to hunt three empuses. The key is attacking enemies when they are in a group to deal as much damage as possible before they disperse. Another thing you can do is to detonate the current prosthesis and use it up for a strong area attack (e.g. the Gerbera comes in quite handy).
When you have completed the task, you will receive a fragment of a blue sphere.