This secret mission occurs in the large sewer chamber where Dante acquired the Rusty Key in Mission 06: Evil of the Waterways. To access it, Dante must have already picked up the Blue Orb Fragment in the dead-end area near the start of Mission 6. It can be accessed in either Mission 07: Holding the Key of Ardor or in the interval before the "official" start of Mission 08: The Legendary Knight Returns. It is best to do it in the latter, as the mission involves copious amounts of waiting, which is not a good idea while holding the activated Guiding Light.
The mission tasks Dante with luring one of the two Kyklops in the room into destroying the other with its projectile attack for a Blue Orb Fragment, and he is not allowed to deal any damage himself. Only one of the two creatures has to be defeated in this way for the Fragment to appear, after which Dante is free to kill the other one.
The principle issue in this room is that the fixed cameras are really not designed for fighting Kyklops, making it quite hard to keep track of both of them, particularly during their leaping attack. It is best to try to stand so that one Kyklops is between Dante and the other Kyklops, rather than trying to stand between them and evade their projectiles. The creatures will not check if their companion is in the way before firing at Dante.