The mannequin should have been revived in the previous step and Rushmore will now tell you to find an Elemental Orb around the map. Find the orb and escort it through the map and bring it to the mannequin in APD Interrogation.
After you have fully prepared with your preferred loadout, head down to the computers in the APD Control area and have every player interact with them to start the boss fight.
Hint: go to the next step and read it first so you know what you're up to