Infiltrate the palace and locate the King's office to find and defend Napoleon Bonaparte.
Assassin's Creed: Unity guide - Sequence 8 - Memory 1: The King's Correspondence - Napoleon
You need to be proactive to get the three cover kills in this mission, so take every opportunity you're afforded and get it out of the way early on. If you're good at cat and mouse or with cherry bombs, any lone guard is fair game for a quick and dirty cover kill anywhere cover is available. I'll guide you to three easy ones, but why not pip me at the post?
- Sabotage three alarm bells
- Perform three cover kills
Infiltrate the Palace
This mission kicks off very close to where you'll enter the Palace, so it's tempting to just move to the south west and climb through the window marked with a platform - but doing so will expose you to a huge number of guards, so let's make it easier and find a better route.
Resist temptation and run south along the roof you're on, so you're out of sight, then cross the street to the building to your south. Head west and you should encounter just one guard on your way to the taller building here. Climb up and take down the two guards on the west side of the courtyard.
If you want to disable the alarm bells, now's your opportunity. One is just to your east, while the other two are further to your west; you can see them on the map screen if you're having trouble spotting them. Getting to them safely involves taking down a lot of guards, but if you have the double air assassination skill you can thin them quickly. Try and get some cover kills for the other optional challenge, too.
Whether you took out the bells or not, the rooftop above the two courtyard guards I mentioned is the place to be when you're ready to move on. Move onto the balcony and use an air assassination on the guard below you, then check for more guards with your Eagle Vision before heading straight through the window.
Find the King's Office
Go into stealth mode as soon as you enter, obviously. If you turn right and head into the next room you can take wait for a gap in the guard's patrol cover behind a couch; this is a perfect cover kill opportunity if you're yet to achieve the necessary three.
Keep an eye out for a gap in the brute's patrol as you make your way back to the previous room. When the brute goes back into the room you just exited, kill him - if you use the doorway as cover, there's your second cover kill. The other guards in the next room won't see you, and you can now kill him at your leisure or just move on.
Now you need to move north. Use Eagle Vision to check the guards' patrols, then go down and back up the stairs ahead. You'll probably need to kill the guard on the landing in the open, but the next room offers the chance for your final cover kill with a small group of enemies.
If you use Eagle Vision you can see a blue and red enemy ahead, as well as a brute off to one side. If you get close they'll start fighting, but you can avoid them and lockpick the door on the right. This allows you to take out the brute before the two adversaries end their confrontation, which makes taking out the victor much easier. Enter the Kin's office to meet napoleon.
Find the Iron Cabinet
This is a very easy objective to complete if you remember to switch on Eagle Vision. It's the big glowing cabinet. Interact it with it to advance the plot.
Several waves of baddies with arrive to bash you up, but there're no particular challenges. If you're having trouble, smoke bombs and stun grenades are a great help, and a quick pistol or phantom blade is always a comfort.
I've not tested this theory, but I believe if you immediately take cover at the door between waves you may be able to get a cover kill from each new wave as it enters - but hopefully you got them all already.
Once the battle is over, you just need to walk with Napoleon, and finally watch a cutscene, to complete the mission. Personable chappie, isn't he?