Once you reach the first Divinity Statue in Main Mission 15, you will find a slightly hidden path behind it. This will lead to a series of hills that you’re supposed to climb with the Punch Line’s Jet Gadget technique, although you can also get to the top with the Gerbera’s dodge maneuver. Once you reach the top, there should be a little alcove here that seems like it overlooks the area you just traversed. The red glow for the Secret Mission will be there.
The Gerbera makes this Secret Mission a lot easier than without it. You have to get to the end within a certain time limit, which can be rather strict by doing the challenge normally. Start out by running forward and jumping on the lowest platform, then perform a wall jump to get on the lowest spot of the platform just above you. While you can go up the ramp here that looks like the way to go, ignore it and turn around to spy another ledge that’s easily accessible with a double jump. From here, it’s a simple jump to reach the path above you, where you will want to run through the tunnel to the next section.
By far, the next section is definitely the most annoying part of the Secret Mission, as you’re dealing with the collapsible platforms from an earlier mission. To begin, jump on the stable platform just ahead of you and while you think you’ll be getting on the first platform, you’re going to skip it. The reason for this is that the collapsing platforms are not in your favor, as they drop in an order that doesn’t benefit you. Instead, you’re aiming for the second platform on the right side. Double jump and use Gerbera to boost onto it, then quickly use the third platform before it collapses. From there, it should be one more double jump and Gerbera boost to the end.