You can earn this achievement in Griffith Park, which you can access late in the game in Hollywood. To find Griffith Park (which leads to Bastion), travel due north in the main Hollywood map. A computer there will talk to you. Ignore that bastard Matthias for the purposes of this achievement. There is a mound of bricks to the left of that spot. Use a grenade or brute force to clear the way to Griffith Park.
Once you make it through there, talk to Ascension McDade and accept his quest. Then travel a little further north from there to a spot with a medic and weapons vendor. Head right and speak with Fealty Mayweather. [In this room there is a case you can examine. If you have a Perception skill of 10 you can collect a book here that will unlock the "Blast from the Past" achievement once read] She will give you a quest. Return to the vendor room and head north, speaking with Retribution Jones in the chapel. He will also give you a quest to find evidence. Now about face and return to the room with the two vendors. Take the door outside.
From there look to the right for a fence with an eyeball near it, but on the other side. Use your locksmith to grab the eyeball, then return to Fealty with it. She will give you the evidence you need. Give this to Retribution Jones, and after a short cutscene the achievement will unlock.
Once you have control of your characters again, immediately kill Retribution Jones and Ascension McDade. I killed Fealty Mayweather too just to be safe. This unlocks the "Religious Persecution" achievement as a nice bonus.