This mission tasks Dante with killing 100 Phantom Babies scattered around the Cathedral in 30 seconds for a Blue Orb Fragment. It is best to use Ebony & Ivory for this. The Phantom Babies are in six groups: two at the starting end of the cathedral, two halfway along, and two by the altar. The groups in the starting area and by the altar are enough to beat the mission, after which the player can have Dante mop up any stragglers for Red Orbs. The Blue Orb Fragment appears by the exit door if the mission is completed successfully.
If the player is really having trouble, the mission can be completed instantly by using a Holy Water and the game then saved/reset/reloaded to save having the Blue Orb Fragment.
This mission can be used to farm Red Orbs by repeatedly failing it, though the fight with Phantom in the corridor immediately afterwards is usually preferred for this as Phantom gives several times more Red Orbs than this secret mission does.