The first time I did this mission I through I only had to press the action key once, but make sure you open the mailbox and pick up the arm. The time is fairly limited, I finished with 37 seconds to spare, but planning out the route and knowing how to get to each mailbox is important. here is a guide that explains the best routes and how to get to each mailbox:
This quest is actually very easy. First you talk to Nazir who tells you to go to Whiterun to confirm with Amaund Motierre that the assassination is still a deal. Afterwards you go to the Katariah which is laying under Solitude in the ocean.
On Board the Katariah
On Board the Katariah you encounter many of Penitus Oculatus Agents. Your best bet is to use stealth and a bow and arrow as you can catch many guards by suprise or even kill them before they discover you. I found that this made this quest very easy and I didn’t even come close to dying once. Eventually you find yourself on the deck of the Katariah. There are about 4 Pernitus Oculats Agents on deck. Kill them with stealth. Next you go to the Emperors quarters where you find Emperor Titus Mede II sitting behind his desk defenseless.
After you have done the deed report back to Nazir in the Dawnstar Sanctuary.
Full Walk Through YouTube Video
This quest is fairly easy, it consists of a lot of story telling. First you have to kill a number of Penitus Oculatus Agents. They are fairly easy to kill as they are weaker than guards. I even used a bow and arrow in close combat and was easily able to dispatch them.
Inside you find Nazir fighting Arcturus. I found that satying out of the fight and using a bow and arrow to get shots on Arcturus was the easiest way to kill him. Don’t worry about hitting Nazir, he can’t be killed and he won’t turn on you if you shoot him too much.
To escape the fire you get into the coffin of the Night Mother. I am not sure how Nazir and the girl survive, but they do and they pull you out of the water. Afterwards you talk to Astrid and she eventually asks you to kill her with the Blade of Woe.
Full Walk Through YouTube Video
Preparing the Potage Le Magnifique
This quest is actually very simple. You have already got the required paperwork to get inside the palace. In the making of the Potage Le Magnifique you can add anything you want to the dish, Gianna won’t question it since you are the Gourmet. At the end be sure to add the Jarrin Root, it will save you from manually killing the Emperor.
Killing the Impostor of the Emperor of Tamriel
If you have added the secret ingredient (Jarrin Root) the the impostor of the emperor of tamriel will die automatomatically. To speed things up you can use other means of killing him. The only thing that is important is that he dies.
Escaping the Palace Without a Bounty
After you leave the death scene you will run into 4 imperials who will try to put you into prison. They are easy to kill, although I advise against using a bow and arrow because a sword can deal out much more damage per time than a bow unless you are sneaking. You won’t have a bounty after you have killed all four guards. All of the enemies are called “Penitus Oculatos Agent” and they are much weaker than normal guards are.
After you have escaped you have to head back to the Dark Brotherhood Sanctuary that has been raided. At this point the next quest starts.