The shortest and easiest one is most likely Ellie’s. After you’ve recruited her and taken her on a few missions, she’ll be standing by the Unreliables’ exit ramp and will say “can we talk” when she sees you. She’ll want to go see her parents on Byzantium, so take her there and meet her parents. After the lengthy discussion with her parents, you’ll go outside and talk more then go to a location in Byzantium.
Once you discover that location it will lead you off planet. Go there then out of the landing area and towards the marker. Keep a powerful weapon up as this area is filled with large amounts of Outlaws, Rapitdons and Raptidons that spit acid. Fight through, take cover as much as possible while trying to protect your allies, watch out for the pools of acid as the effects linger.
Once you’re through and not dead and have gotten to the area it’s a short walk to the building you need. You’ll either have three options; intimidate, hack the computer in the back or bribe. It’ll be a short conversation and once you’ve completed it before Ellie talks to you thanking you in her own way, the achievement will pop