As the description says you have to complete Act 2 Chapter 1 (Standard or Arcade) on any difficulty. I got mine on Normal.
You will start the chapter with nothing but a pistol, so try to pick up some weapons on your way. As soon as you pass the first containers, enemies will start to pop up from the air. Take cover and move slowly through the right side and as you keep walking you’ll find a crushed helicopter. Next to it, on the floor, is one of the collective items so pick it up. Turn back and go straight and you’ll see the second crushed helicopter and next to it Santiago and Marcus will be lying on the floor.
After a cut scene you will have to fight some Savage Boomers. Take the right side and move slowly. You’ll find several spots to take cover and shoot. As you keep moving you’ll find a siege beast (monster machine that throws balls of fire) get close to it and by pressing X, you will be able to operate it. Point and shoot as you normally would to destroy the enemy siege beasts and a brumak.
After that there is a small cut scene and a checkpoint, you will have to regroup with Anya and Jace, keep left and you’ll find them next to the containers. Next you’ll have another cut scene followed by a checkpoint, and then you’ll have to defend against attacking Locusts. Make sure not to miss the container with the gears logo because on the back of it you’ll find another Cog Tags.
As you keep moving forward you’ll fight another brumak, take cover and shoot him in the face. Keep doing that and you’ll kill him in no time. After that your achievement will be unlocked.
I think this chapter was fun and fair, I’m sure you will not have trouble getting the achievement.
Best of luck.