In Act 4, Chapter 1, you will start in Char, one of the first Cities to be hit by the Hammer Strike. All you need to do for this Achievement is make sure that you don't shoot, or walk into any of the Ash People. Your AI teammates can walk into them and destroy them, but it has to be you that doesn't.
Once you have helped the stranded kill the two Stalks that appear you will be take up the tower to see Griffin.
The person leading you will have an arguement with someone on the other end of a door, which will then open for you.
Once you are inside, keep to the right hand side and you will see a barred door, just before a table. Stand outside of that door and you will hear a woman says "I guess all COG aren't assholes" (Or something like that) and the achievement will unlock.