Start by following the path up, near the edge you'll find the Beowulf weapon. To the right of where you started is a statue, go down there and equip the Beowulf. Head back up and use the gauntlets on the statue to get a Blue Orb Fragment and to reveal the door. Step onto the elevator and ride it up. Proceed through the crack in the wall and into the door. In this next area as you go up the thin hallway there's two blue torches. Jump up above the left one and into the indentation in the wall. You'll find Secret Mission 9.
Here's an easy secret mission, simply set up the Gunslinger style (or don't, it's not a requirement) and start the mission. Enemies will constantly appear on other moving platforms and you have to destroy them before they escape. Fortunately they always come with one of those exploding enemies so target those first with Ebony & Ivory and then just slam the square button to clean up the remainders. The reason I said to use Gunslinger is because it makes the mission so easy, between platforms you can hold the square button to charge up your guns and blast any enemies almost immediately after they appear giving you more than enough time to take care of what remains. Just make sure you start firing as soon as possible on the last few platforms which only have exploding guys, because they move fast, and they don't stop.
Drop down and head into the next room, defeat all the enemies here and leave. Descend the stairs and grab the Green Orb under the statue, then get on the moving platform and take it to the other side. Enter the room and go through the gate. Head through another gate toward a green circle on the ground and beat up the enemies, go back and take the other gate now. Beat up all the horse statues and then afterward choose the gate leading to a shiny object on the other side, it is a Vital Star S. Return to the previous room and head toward the red this time. On to the next one after defeating the enemies, you'll find a Blue Orb Fragment before leaving this area.
Use the statue if you need to and then enter the door on the opposite side. In this area there's a large blue pillar of light which acts as a teleportation device. Once you step into it exit the room via the door with the blood streak and head back toward the direction of where Cerberus was.
