You can't slow down time here. So use another technique. Before initiating the mission, make sure that you have a full Disaster Gauge (Level 4 Gunslinger Style). Ready Trickster, Pandora and Devil Trigger. Use D.T. then Double Jump then Sky Star twice then quickly switch to Gunslinger and use Argument to float to the end of the corridor.
If you are able to float with the Argument very fast (by holding the forward button, and not pressing any left or right movement buttons), only 20% Disaster Gauge will remain. Then switch to a new Combat Style to drop off. Alternatively, you can just use the rest of the Disaster Gauge and continue floating forwards, just in case.
This Secret Mission's Seal can be found in Mission 17, just on the area before reaching the Plaza in Fortuna, a canister can be located in the right side from where you enter the area. Destroy the canister and the seal can be accessed.
A Steeplechase is a horse race where the course was designed with fence jumping, obstacles, and steeples.