This page in our guide to Devil May Cry 5 contains the walkthrough for the last Secret Mission.
Secret Mission 12 in Devil May Cry 5
Where to find it: Mission 10 - Diverging Point: Dante
At some point during this mission, you will reach an arena with Hellbats and a Behemoth. Defeat them, jump down, and eliminate more enemies. Move forward - jump down on the ledge and go to the large Red Orb.
Jump on the platform with the blue sphere and launch yourself forward. Secret Mission 12, the last of them, is inside the small cave.
This is a rather difficult mission - you have to collect 800 Red Orbs within 1:30 minutes. To complete it, buy Dr. Faust 3 skill, switch to Gunslinger style and use spin attacks with the hat to increase the number of Red Orbs you get by defeating enemies. Complete the mission, and you will receive a Blue Orb Fragment.?