Cut the chains of the gate open and proceed to the entrance under the bridge. Once you've reached Elliott's computer room, you can find the Declassify Mission Skull there. Activating it will allow locusts to use one-shots. While the bot is hacking the mainframe, you have to fend off a heavy enemy attack waves and prevent them from reaching the bot. Grab and deploy the nearby gnasher sentinel to protect the entrance leading to the computer room.
If you have the declassify mission active, the Oneshot-wielding locusts will always try to set up in the balcony far ahead. You can encircle them using the corridors to the left or right though you have to go back immediately since the enemies will still target the bot.
There's a COG tag in the balcony on the other side. Check the corner upstairs, above the COG skull in the first floor. This is the same balcony where the Oneshot locusts camp and take their potshots.
After defeating the first wave, the bot will move out in the open and attempt to hack the main terminal. Grab the auto-turret and deploy it near the bot's location to protect it. Before the second enemy wave comes in, you may want to grab some ammo and plant some grenades along the stairs to help you deal with the rushing enemies.
After taking out more enemies, a berserker will appear. Along with some flamers. Take out the flamers first and other minor soldiers so you can grab the Flamethrower later on. Replace one of your weapons with the flamethrower and scorch the berseker. Once it is burned, keep firing at it using your other weapon to deal damage. Once it cools down and returns to its iron-skin state, burn it again with the flamethrower and repeat the process. The mission will be complete afterward.