Story related, can not be missed.
*** Spoiler - click to reveal ***After finishing the Earlston Yellow Zone, you will head to the subway.
From the subway, you are directed to a preliminary ambush against the KPA.
Defeat the enemies outside, head through the bus to the trucks, defend the trucks against enemy waves, and return to the trucks.
After this is over, a cinematic will play, and you will have to deal with a goliath.
Hit it with rockets until it dies. There are plenty of ammo caches around.
On the upside, destroying the goliath will also unlock:
After it is defeated, rip out it's brain, leave the green area on your map, and the achievement will unlock.