DiegoTan's guide is great and shows how to navigate through the conversations successfully. I just wanted to highlight that you can use cheats to quickly get to those NPCs and breeze through this achievement.
enables noclip. You can use this to breeze through obstacles like locked doors or to fly up to high areas. Just know that it's a lot slower than your normal movement, you need to put in the cheat again to disable it and interact with the world, and if you fly past a loading barrier the game won't load it in, so it's best to only use noclip for brief periods to get to where you need to be.
The rest of the guide here since it's
spoiler-y:
*** Spoiler - click to reveal ***I did it on a save where I had already talked to Khabash and Naevia in previous loops, and already put all four plaques back on the Obelisk. But I'm guessing you could do this on a fresh save too and it would work fine.
For Khabash, go to the temple of Demeter (it's on the same block and side of the street as Georgius and Virgil's shops) and crouch through the wooden door in the back and follow the path. You can ignore the philosopher sitting there, as you can noclip through the gate instead of getting the key from him. From there, follow the way until you encounter Khabash. The achievement is about your second conversation with him, so just have the first then jump down and follow the path until you encounter him again.
For Naevia, go to the palace, which is the building up high on the way to the Grand Temple that has a note from her on the front door. Above that front door and a little to the left there's a balcony with a zip line coming from it; you can use noclip to fly up to the balcony. From there just head inside; when you see the statue on a rack you should get pulled into a conversation.
For the God of the Underworld, go to the Grand Temple, up at the highest point of the city with the obelisk out front. If you don't have the plaques, you should be able to noclip through the first door and then normally work your way through the rest.