Declassify Mission: Elite Locust defend Karn
The last COG tag of this chapter is located dangerously close to Karn's position. From your starting position, run to the left until you see a COG tag skull on the wall. The COG tag is located on the floor almost in front of the skull.
If you have activated the declassify mission, Karn will be reinforced by a variety of high-tier locust units. Karn has two primary attacks; his dual mounted mulchers and a rain of fiery napalm. First, shoot its mini-guns to force Karn to stop and launch the napalm canisters. While it's attempting to launch the canisters, you can shoot the fiery part of his mount's back to deal damage. Don't bother attacking Karn directly. You must attack the parts that turn red in your crosshair.
Once you've blown out the napalm launcher on his back, he'll temporarily leave the battlefield and summon his reinforcements. Take them out quickly before he returns to the battlefield. Once he returns, he will have two new attacks; a burrowing mine and a charging attack. The only way you can damage him is to wait for his mount's leg to glow red – this means its preparing to launch the burrowing mines.
After wounding him the second time, he'll leave again and release nemacysts. Take cover and wait until he summons a second wave of reinforcements again. Depending on his reinforcements, you may want to stick close to at least one of your allies so you can be easily revived in case you go down. Prioritize taking out the minions first to make your job easier.
For the last phase, you have to take out mounted mulchers and shoot for the beast's mount as it launches some nemacysts. This will gravely wound it eventually and while it's writhing in pain, keep shooting at its head to finish it off.
Congratulations! You've finished the campaign!