On Mission 11: Reason, after destroying the first Blood Clot at the building area, instead of going down, go back up and climb up to the new location you just unlocked. The Secret Mission and pattern is at that location.
The goal of this Secret Mission is to maintain a Stylish Rank of S or higher for 50 seconds. Due to this requirement, the mission will immediately fail if Dante takes a hit.
Regardless of what the player intends to do, the best way to start is by performing an Air Taunt, which will immediately raise the Style rank to SS. One of the better weapons to use to maintain Style is Balrog, alternating between Blow and Kick mode to keep varying up combos.
Another method is to use Ebony & Ivory with Gunslinger Style, firing one gun until Style stops going up, then the other, then using Twosome Time and finally Honeycomb Fire, using Trickster Style to evade if any enemies get close. This method does, however, require Gunslinger at level 4 in order to have all the relevant moves. If the player is returning to this mission from later in the game, Double Kalina Ann's Mega Cascade can also be very effective at maintaining Style.