This one is part of the main story so
***** SPOILER ALERT!!!***
Okay, So to get his achievement you have to complete the main story quest to infiltrate the Thalmor Embassy, called "Diplomatic Immunity"
It does not matter how you complete the mission, so if you want you could go berserk and kill everyone.
But for those of you that wants to know the sneaky way of doing it, here it is.
Follow the instructions given to you and go to Malborn in Solitude. Bring with you sneaking equipment (The Dark Brotherhood Boots and Gloves are highly recomended.) A bunch of invisibility potions and a couple of weapons of your choice. When you get to him he will explain the plan to you and also say that you can deposit items with him that he will bring into the embassy for you.
It is important that you give him EVERYTHING! that you want to bring with you into the mansion, because when you continue everythin else will be removed from your inventory untill the quest is done.
After depositing your equipment go and meet Delphine near solitudes stables. She will give you an invitation and party clothes. Equip the part clothes and tell her you are ready. She will then take you to the embassy. When you get there. Show your invitation to the guard and enter the mansion.
After talking a little to the Thalmor ambassador, talk to Malborn, he is working at the bar. He will tell you to create a distraction. Ask him for a drink and take that to a guy named Razelan (you may have spotted him in the courtyard when entering tha mansion.
He will ask for a drink, Give it to him. Then ask him to cause a distraction. He will do so, and you should go talk to Malborn. He will let you into the kitchens and lead you to a chest containing your equipment.
Start going through the mansion. In the first room you get to you can overhear a conversation between two guards. Kill them any way you see fit or sneak past them. Killing them will not alert any other guards. After getting past these. head east and open the door to the exterior courtyard. Now it's time for those invisibility potions to do their work. First create a distraction. Send a fireball or something to make the guards leave their posts.
Then drink your invisibility potions and run across the courtyard to the other building (elenwen's Solar) and enter it. Once your inside you can overhear a conversation between a guy named Rulindil and another fellow. They mention an interrogation room downstairs. Sneak into the Northwest office using another potion or time your run with the guard patrolling the floor. The room your looking for is next to Rulindils study. You know you are in the right place when you find a small chest containing some dossiers on Delphine and Ulfric Stormcloak, a report on the dragon attacks and a key to the interrogation chambers.
Now, there are two stairs leading down to the dungeons. You can use both but if you still want to go sneaky, use the one in the Northeastern part of the area. This takes you to a balcony where you can see a single guard patrolling. You could also kill Rulindil and steal his key and go down through his office.
If not you can now see him go down to the dungeons and start interrogating the prisoner.
Anyway.. Sneak down and kill the single guard (and Rulindil if you haven't allready and talk to the prisoner. He will tell you that he have mentioned the name Esbrn in Riften. Additionally you could search a chest in the room that holds a dossier on Esbern. Reading that along with the dossiers on Delphine and Ulfric will also progress the quest. Free the Prisoner and two Thalmor guards will appear dragging Malborn with them. If you attack and kill them quickly Malborn will survive, it does not matter for this achievement if he dies.
No matter, kill the two Thalmor and grab a Trap Door key from one of the bodies (or the guard you killed earlier and escape through the trapdoor located in the far end of the dungeon.
Down there you will face a Frost Troll. Kill it (if you wait you can kill it before dropping down in the chamber with magic or ranged attacks.) and leave the dungeon.
The achievement popped for me when i did that the first time i played through, but on my second playthrough it popped after completing the quest.
Anyway. fast travel to Riverwood and speak with Delphine (The inventory items you deposited with her earlier is in a chest next to the wall in her secret room.) She will tell you that you need to go to Riften and find the guy named Esbern who is a Blade Archivist.
The quest is completed and if you didn't get the Cheevo earlier it should pop during or after this conversation.