Continue until you find the section's Declassified Mission skull. This objective will require you to finish the section in 4 minutes before the deployment of the Academy's proximal poison gas defense system. Grab some ammo scattered around or even take the Boomshot / Scorcher from the cabinet. When ready, open the door to the next area.
There's a Mulcher mini-gun to the stairs to the left. Grab it and take cover as you mow down boomers or grinders. There's also a One-Shot rail gun randomly located in the right side of the courtyard. Clear up the courtyard as fast as you can especially if you have activated the Declassify mission.
Once all enemies are taken out, a Rager will burst out of the door. Aim for the head as it charges to take it out. You can also kill it instantly by using heavy weapons like the Boomshot. Continue inside and clear the building. You'll find a COG skull on the wall to the right. The COG tag is located inside the room to the left, before the hallway.
A. Sullivan, SSGT, Andius
Continue downstairs and kill/ deliberately detonate the tickers along the way. More ragers and elites are waiting outside in the narrow area so make sure to evade and take cover. Let your teammates take the brunt of the ragers charge. You have to make it quick; otherwise you won't have enough time to escape. Once you cleared the area, immediately head to the lab entrance and press up to continue.