On the opposite side of the room from Sophia is a large screen. Below the screen you can place down the trophies you have collected. The button in the middle should now be green, and interacting with it should start a new random, uncompleted challenge. During each challenge, zombies will spawn infinitely. If you fail a challenge, you must wait until the next round and go back to the button to activate your challenge again. You will need to complete all six challenges to head to the boss fight.
NOTE BEFORE STARTING A CHALLENGE: The Bomb Challenge will present to you a sequence of locations on the map that you must note the order of, so read through that challenge description first. You should have a camera ready or be able to note the order quickly as failing this challenge will instantly kill any players in the game even remotely near the bombs around the map, which can potentially end the entire run. You can fail it by running out of time or deactivating bombs in a wrong order.
When initiating a new Challenge, lights will circulate between the trophies before settling on one. Based on the trophy and Sophia's instructions you can figure out which Challenge must be completed.
The Challenges based on trophies from left to right are: