During Main Mission 09, you can break a wall down by summoning Nightmare, which will have a Gold Orb floating behind it. Not long after that, there will be an open area that involves a fight with a Proto Angelo, Pyrobats and a Baphomet. Defeat them all and look around the arena for a doorway that has some roots blocking it. While standing right next to this, look behind the roots to spot a blood clot on the other side. Summon Nightmare once again to have him destroy the blood clot, nabbing the Blue Orb Fragment in the open. There are some "stairs" behind this fragment, so climb to the top to find the red glow and look back at the entrance to start the Secret Mission.
Twenty seconds might not sound like a lot of time to defeat all of the demons in this Secret Mission, and it really isn’t, but you only have to go through a single Qliphoth tentacle and two Hell Caina. This strategy involves using Nightmare for it, as his quick and heavy damage will make things a lot easier on your end. As you walk forward at the beginning of the mission, you only have the tentacle, but the Caina will spawn behind it. You will want to run forward and use Griffon or Shadow (the former’s Flank Attack will work wonders) to destroy the tentacle first. As soon as you do this, hit L1/LB to summon Nightmare to drop in on the Caina. From here on out, it’s just a matter of depleting the rest of their health to zero and finishing them off with your cane in time.