This achievement is missable
Just to be clear to get this achievement you need to complete two quests: 'A Deadly Plot' and 'Reason of State' the first quest 'A Deadly Plot' will not DIRECTLY be followed by 'Reason of State' So After completing 'A Deadly Plot' you will need to continue the main questline!
Here's the "how to" if you're having problems or don't mind parts of these quests being spoiled. There's even a video that talks you through it to clear any and all confusion about the trophy.
*** Spoiler - click to reveal ***
I will divide this guide in two parts: Part one will focus on the first time in Novigrad, meeting Dijkstra and how to trigger the first quest 'A Deadly Plot'.
The second part will focus on the second time in Novigrad, finding Philippa and making the right dialogue choices with Dijkstra to trigger 'Reason of State'
Part One: 'A Deadly Plot'
*** Spoiler - click to reveal ***While playing the main questline in Novigrad you will meet a man named Dijkstra. Dijkstra won't give 'A Deadly Plot' right away. Just play the main quest until you burn down the witch hunters prison. After this quest you will have a talk with Dijkstra, this talk does not matter. (So don't worry if he is pissed because you have not recovered his treasure) The important part is Triss asking you for help. Make sure you help her! (complete her side quests) After you have romanced Triss or have not. Dijkstra will ask you for help. Agree to help, this will trigger the quest 'A Deadly Plot'. This quest is Pretty easy. (Don't forget to play Thaler if you are going for Gwent stuff) Now when you finish the quest you will need to progress the main questline. So find Ciri battle the Hunt at Kaer Morhen and go to Bald Mountain. After all this, you will return to Novigrad.
Part two: 'Reason of State'
*** Spoiler - click to reveal ***So when you return to Novigrad you will get the quest 'Final Preparations' this is a big quest with multiple objectives/side quests. When helping Triss find Phillipa your search will lead you to Dijkstra's bathhouse. After getting Philippa, Dijkstra will approach you and ask you to hand over Phillipa. You will need to solve this without violence. Dialogue needs to go: I need her > Tell you what the Emperor is after > Heard it from Emhyr himself > Let her talk. After this talk the quest 'Reason of State' will begin. This quest is also straightforward. Finish this quest and the achievement will be yours.