On Mission 09: Genesis, after the second forced battle, go to the left side rather than going down on the right. There is one Blue Orb Fragment in the distance, locked by some Qliphoth roots. Summoning Nightmare will make him destroy the Blood Clot inside this area which opens up the passage for V.
If the player has no Devil Trigger to summon Nightmare, it is actually possible to manually line Griffon up to fire his basic shots through the gap in the roots and destroy the Blood Clot that way, though this is extremely fiddly and not recommended.
After obtaining the fragment, climb up the platform right in front of it, the Secret Mission area is up there.
Using V's Royal Fork Skill is a great to beat this Secret Mission, firstly, kill the Qliphoth Root that spawns in front, then, as you command Shadow and Griffon to attack the two Hell Cainas, start using Royal Fork in order to deal extra damage. Once the enemies are either at low health or in their "white" state (ready to be finished off), they will instantly die by Royal Fork, which is faster than using the regular Checkmate or Gambit moves with V's Cane.
It's also possible to beat this Secret Mission by spawning Nightmare and using Checkmate/Gambit, though it may require precise timing due to the 20 second limit given.