During Main Mission 16, you will come to a point where you can either go straight ahead and down a giant hole. Opt for the giant hole and go all the way to the bottom to fight some Chaos and a Fury. After winning, there will be another giant hole, but you want to be careful not to fall to the bottom here. Jump down to the first ledge below you, then look for another ledge below that one that has some Red Orbs on it. From here, you should be able to spy a large Red Orbs Cluster across from you, which is where you want to go. Pass through the opening behind and drop down to the ledge below, then look for another just under that. This should have an alcove, which is where you’ll find the red glow and the Secret Mission.
Your goal is to somehow get 800 Red Orbs out of a single Empusa, all within 90 seconds. Naturally, the Empusa doesn’t cough up that many orbs, so you’ll have to find another way to do it. If you didn’t know, a normal Empusa can upgrade to a Red Empusa by sucking up some blood, and there are actually two small pools of blood in the Secret Mission. So, you’ll need to lure over the Empusa to some blood to make it turn into a Red Empusa, which is the first step.
After doing that, switch to the Faust weapon, as well as the Gunslinger style. Use the basic action here (where Dante spins around and throws the hat in a circle), but make sure you don’t trigger Hat Trick, should you happen to have that skill. Keep doing this skill until you eventually kill the Empusa and get the necessary Red Orbs, finishing the Secret Mission.