After solving the blood fountain "puzzle" and opening the path below it in Main Mission 12, follow the path until you jump up some ledges. As soon as you reach the top, turn around to spy another ledge a little bit in the distance. You can double jump towards the ledge, then use the Trickster dash to reach it, but it’s also possible to use Stinger near the edge to get a long jump to help reach it easier. Once you reach it, you will find the red glow for the Secret Mission here.
Your objective for this Secret Mission is to stay in the air for 15 seconds, with the counter beginning as soon as you leave the ground. There is a Hell Antenora and a Pyrobat to join you, but you shouldn’t need either for this challenge, other than a simple target. The Cavaliere will be your weapon of choice and you will definitely want the Highside ability for it. As soon as the Secret Mission begins, wait for the Pyrobat to spawn and lock onto it. From there, jump once and begin using Highside with Cavaliere (hold R1/RB, then press back on the left analog and Triangle/Y).
If done successfully, then you should get some air from doing Highside, so continue doing it until you start to lose altitude. At this point, you should still have your double jump saved, so use it before you hit the ground. To add onto that, you also have the Trickster air dash to add another second or so onto your time. Should you manage to snag an enemy with Highside, you can hold the attack button down to keep yourself afloat even longer.