This Challenge always comes last after the five others.
Under the trophies a drawer will pop out with a keycard.
Pick up this keycard and all players should go to the house. On the ground floor there should be a red computer screen near the toilet. Placing the keycard here will start a lockdown.
An infinite number of Manglers will begin to spawn in during the lockdown, and you must survive while the computer downloads. You should see its progress on your screen.
After completing the lockdown, pick up the keycard from the computer and return it to Sophia. She will give you a power core and leave the map.
Pick up the power core and return to spawn. Using the Gauntlet of Siegfried, release the Whelp and it will bring the power core to Nikolai. You will be able to begin the boss fight now.