In this mission, Nero must prevent Scarecrow from being taken over and must eliminate all enemies.
Wait for the Scarecrow, execute Hold, get up on the cliff you came through into this area in Mission 07, the one near the Gyro Blade, leave it here, jump down and Buster all Chimeras. Then destroy the Scarecrow and get the Blue Orb Fragment.
A much easier strategy with Dante is to charge up the disaster gauge to about 70%. As soon as the mission starts, use PF666: Omen. Since the scarecrow has more health than the Chimeras, you'll kill all the Chimeras while leaving the Scarecrow with a bit of health left over.
This Secret Mission's Seal can be found near the Security Corridor, after moving safely from the large Laser Barrier's look to your right, the seal is just on the wall.