Located in Main Mission 11, you will journey through a destroyed building after breaking a blood clot just outside of it. Inside of the ruined building, you will find more blood clots, which will make the one building sink. Do this one time and find the building that sunk a little bit, which is the one you passed through on the way into this area. There is a section above the ground floor one that you can double/wall jump to, so do that now and look towards the one corner for the red glow, gaining access to the Secret Mission.
When you begin this Secret Mission, you will have a Stylish Rank of S and must maintain it until time runs out. You are given 50 seconds and a bunch of enemies, but you will want to be extremely careful to not kill them all, as no more will spawn. There will be three Hell Cainas and two Hell Antenoras to fight and you must make sure you do not get hit at all, since that will tank your ranking below S. Using a variety of attacks is ideal here, since the Stylish Rank needs to be kept high, so make sure you’re not doing the same attack over and over.
The Balrog can be one of the better weapons for this, since it has two different modes for a wide array of attacks. There’s both Blow Mode and Kick Mode, plus triggering Ignition on either one will help in maintaining your Stylish Rank. Don’t forget about your ranged weapons, too, as well as your various styles, since you gain access to new moves with all of your weapons. It may take a bit, if you’re unfamiliar with Dante, but pushing enemies away from you is also a bad thing here, so try not to use Stinger too much, either.