Shadowrun: Hong Kong - Extended Edition

To unlock this achievement you must complete the optional quest "Outsiders" which is obtained from your computer in the 'Bolthole', shortly after becoming a shadow runner during the main storyline. The mission doesn't completely hold your hand, but it is fairly simple to complete. I will keep my guide vague for those who want to keep some mystery to their game, so please keep this in mind.

Once the quest has been accepted from your computer, travel to the train station (you used the train station during the main storyline) to move to the quest area. After following the first few quest markers, talking to your quest givers, and viewing the latest murder scene (making sure to exhaust all search icons and dialogue), head back into the street and make sure to talk with every NPC in the surrounding area - the outside map is kind of 'O' shaped so just chose a direction and talk to everyone in the circle before moving on. Three NPCs should mention different information about an Elder named Magpie amongst other things, another street NPC informs you of a red pole gang in the car park (top right of the 'O' shaped map).

With this information, return to the quest givers/elders to gain Magpie's store key which I think needs a small charisma check or an etiquette check (please can someone confirm?). Heading back to the street, head to the right side of the map, enter Magpie's shop & Investigate inside fully. Heed Duncan's warning (if you brought him) and don't inform the Elders of your discoveries until the conclusion of the quest.
Next, back on the street, head to the car park, & enter. The gang inside will freely give you some information to do with the current murder scene as well as mention something about the storm drains. You may avoid bloodshed with the 'gang' or 'socialite' etiquettes to peacefully search further into the car park for vital evidence. Without these etiquettes, you must fight your way past the gang, in which case I suggest pulling your team back and funneling the enemies towards you as they can pack quite a punch if you face them head-on early game. Once the back of the car park has been fully examined, leave the area & talk to the NPC street vendor again (near the car park) about the evidence you found in the car park and he can give you some insight. Return to the area where the quest givers/elders are, go past them and into the neon-looking clothes shop at the top of the screen, and ask about the evidence you just gained for further info.
In the room next to/above the clothes shop, is an arcade area with an NPC called Rainbow, if you have been talking to everyone she should have an option to ask about the storm drain keys. You can also ask Porter Lam (The man outside the current murder scene) for the key instead.

Enter the storm drains near the top of the outdoor map, near the car park, and search the area upwards, before heading downwards to make sure you investigate a certain grisly scene. Eavesdrop on the conversation between the merc group and then converse with these new NPCs to gain further information and a stockroom keycard. The fight here is avoidable so long as you searched the upper area first and you choose the right convo choices. The "Hope & Goodwill" dialogue option leads to a fight if you are gunning for it.

Once you are back on the street, head to the stockroom (Right of the car park). There is a plate of food set out on a table which I think is related to another achievement, so make sure to eat the food for a possibly linked achievement. You will soon be confronted by Gaichu, a rather uncharacteristic ghoul who retains a lot of his memory and function. Listen to his story and agree to confront the Elders with him, to hear their side of the story. You will wait until Nightfall then manually travel directly back to the Elders/Quest giver's location.

You must now lay out all your findings and evidence, accusing the Elders of betraying Gaichu. Make sure to choose all options from top to bottom in that order, choosing the last option before any other will move the conversation on without you presenting all your evidence. If you found enough evidence during the course of the quest, you will convince Porter Lam that the Elders have been doing something wrong and he will arrest them to investigate further. Stop Gaichu from protesting, asking him to stand down, as the best outcome is if the Elders live. If you did not gain enough Evidence then you will fight the Elders leading to a worse outcome.
Once you leave the area, Gaichu will ask why you stopped him, I chose the option about reputation and he happily accepts your reasoning. At this point, the achievement should pop!

Please give feedback for any negative downvotes, I've written this largely from memory from 2 nights ago so I will happily correct any misleading information. :)