This Secret Mission is located in Mission 02: Qliphoth. It is found in the Il Chiaro Mondo Hotel in a bedroom after some Qliphoth roots block off a corridor and smash a hole in the wall, and is announced with a tutorial. The only issue this may give a novice player is that they have to have Nero stand on a glowing red point on the floor between the bed and the wall: after finding this, stand on it and align the symbol to get a prompt to start the secret mission.
This mission's goal is just to kill all the enemies within the time limit, with no special stipulations. The enemies are low-level and clearing the mission should not present any issues: if Nero has Overture equipped it will make particularly short work of them.