On Mission 05: The Devil Sword Sparda, after destroying the Blood Clot at the ship area, there will be a forced battle with one gas tank in the middle. After this battle, the player is supposed to destroy another Blood Clot, instead, head towards the wall which has a yellow waste container nearby and spawn Nightmare, he will break this wall upon being called.
The location ahead has another forced battle, after all enemies are dealt with, search for a set of stairs in this area - this will lead V to the Secret Mission area.
The goal of this Secret Mission is to kill all enemies without taking a single hit, the only enemies present are four Hell Cainas and three Hell Antenora, because of this, a good strategy is to put these enemies in the air and combo them so they stagger and don't attack V. Both Griffon and Shadow are able to place enemies in the air.
In addition, if any of V's Familiars get hit, the mission will not be considered failed.
Before finishing off an enemy with V's Cane, examine the situation and act accordingly, using the Royal Fork skill allows V to finish off all enemies which are low on health at once without being open for attacks since he teleports up in the air, and finishing off an enemy who's in the air will also lower the chances of V getting hit during the animation since both enemy types do not have attacks that reach too high up.