The player will immediately be surrounded by ten Chimera Seeds. A single Scarecrow (Leg) will spawn immediately to the player's right. The player will need to act fast, or the Chimera Seeds will immediately lunge at both the Scarecrow and the player.
Grab the Scarecrow with Hold as soon as possible. The Chimera Seeds can still merge with a Held Scarecrow, so use your jumps and Air Hike to retreat to the cliff behind you (the cliff you came through into this area in Mission 07). Leave it here, then jump down to kill all the Chimera Seeds. Using Snatch and Buster is the safest and fastest way to destroy them.
The Scarecrow may wander back down the cliff to attack the player, if left too close to the edge. The player may only destroy the Scarecrow if the Blue Orb Fragment has already appeared.
Dante's Pandora is likely the only method to kill the Chimera Seeds fast enough. Charge up the disaster gauge to about 70%. As soon as the secret mission starts, use PF666: Omen. Since the Scarecrow has more health than the Chimeras, you'll kill all the Chimera Seeds 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.