SPOILER ALERT: This is a walkthrough for the assassination of Robert de Sable, the ninth target. If you haven't completed that mission yet, I recommend you turn back now.
As the Stylish Assassination series comes to a close, let's review the recommended body count from the first eight assassinations. By "body count" I mean the number of people Altaïr kills by his own hand including the target, pre-assassinated guards and dead body distractions (but not Save Citizens or other side missions).
So we're doing a good job following the tenets of the creed, staying the blade and all that. Robert de Sable will be protected by his Knights Templar at the funeral. Surely this figures to be a bloodbath. However, I'm going to show you a way to complete this assassination without killing anybody. Now that's doing it with style!
INVESTIGATION
All four Pickpocket/Informer missions turn up maps of the cemetery. Here they are in their original orientation, conveniently rotated so that west is up :
The first one shows you where the guards will be stationed during the funeral, so you can sneak up on Robert de Sable from the left and... aw, who am I kidding? The funeral will be an ambush and the Investigations are all bullshit. Just pick any three and be done with it.
The fastest way to complete the Investigations is to start with the Eavesdropping in the northwest corner and then proceed south to the Informer. (Tip: One of the targets is on a roof.) Finally, head east and do the Pickpocket on the way back to the Assassin's Bureau. In my opinion, these are the easiest three Investigations. I've indicated their locations in green on the map further below.
PREPARATION
Here's something useful you can do during your bogus investigation: Save Citizens. Save the citizen nearest the cemetery, and save the next citizen nearest to her. Save citizens in the nearby Poor District and all the way back to the Assassin's Bureau.
Choose a Vigilante location where you'd prefer to fight. My favorite is the one on the other side of the building behind Templar #2 because there's plenty of room to work. Learn the shortest route from the cemetery through a different set of Vigilantes to your preferred Vigilantes. Practice running this route back and forth without using your GPS.
Ordinarily I advise against using Vigilantes during an assassination because it's not sporting. This time I'm doing it for the guards' own protection, so nobody gets hurt out there.
APPROACH
The cemetery is defended against a ground approach by guard posts at both gates. The goal of a zero body count rules out fighting the guards or using a dead body distraction. That leaves two options: Scholars or rooftops. There's supposed to be a group of Scholars at the south end of David's Citadel that will get you through the gate (as shown on one of the Investigation maps above) but, as Spartytime pointed out, they're missing. You can still climb the building where they were supposed to be and jump onto the large railing that runs along the front of the cemetery.
My preferred approach is to jump rooftops and ignore archers. As long as you keep moving and don't bump into them, the archers won't bother you. Once you reach the rooftops within the cemetery's perimeter you'll be out of their range.
BLOODLESS BATTLE
Usually it doesn't matter much where you stand during a cutscene. This time it's different. During the funeral, move as far as you can towards the exit leading to the route you practiced during "PREPARATION". I prefer the north gate because it's only a two man guard post. Besides, there's a guard patrol outside the main gate and I don't want them to join the chase.
As soon as the Imam points you out, run like hell along your practiced route. There will be more foot traffic this time, so use your Tackle ability. Sprint or Free-step when the crowd thins to put distance on the pursuing Templars.
As you pass through the first set of Vigilantes they will grab up to six members of the posse. Don't stop there. Continue to the second set of Vigilantes where you will make your final stand. These Vigilantes do great work. When a guard slips out of their grip they will chase him down and grab him again. I've seen a Vigilante move several city blocks out of position to stay on his man.
I'm going to dispense with the "Robert de Sable" talk now and call your target by her real name, Maria. She's the one with the long gray cape. I sometimes have trouble picking her out of a crowd so I turned on my GPS for this part of the mission.
At this point, Maria should be in the grasp of either the first set of Vigilantes or the second. If she's with the first, wait for the for the second set to get a full complement of six guards (you can help by "steering" them into an available Vigilante) and then run back to her location when the coast is clear.
It's impossible to target someone while they're being held by a Vigilante. This is actually a blessing as it allows you to keep your Hidden Blade armed while moving freely to avoid any loose guards. Find Maria and move behind her. That's important: you can't do a low profile assassination from the front. Move in behind that big gray cape and gently stab her in the back.
Another thing that might happen is you'll meet Maria halfway between the Vigilantes, on the loose. This is trickier to handle without collateral damage. Enter fight mode but don't Counter-attack anybody except Maria. When other guards attack just Deflect (with a sword) or Dodge (if fighting with the Hidden Blade). Maria tends to taunt frequently with squeaky cries of "Death to the Heathen!" and such. If she does that you can jump her with the Hidden Blade. Otherwise, wait for an opportunity to Counter and finish her off.
In the Memory Corridor, Altaïr discovers that he just assassinated Robert de Sable's steward Maria. As she was not his target, he tells her she's free to go. And that's how you achieve a body count of zero. "Mission Accomplished"TM
That raises an interesting plot hole. How can Altaïr spare Maria's life in the Memory Corridor after he just carved her up on the street? The other targets all seem to have bled to death in the Memory Corridor. All except Majd Addin, that is, who required a second jab in the neck.
ESCAPE
Altaïr just spared Maria. You'd think the least she could do is call off her Knights Templar but no. You still have to escape, just like after a successful assassination. In the video I make a mad dash that starts in the Middle District, dips into the Poor District, and ends at some Scholars in the Rich District. These are the same Scholars I used after assassinating Majd Addin. I thought it was appropriate to meet up with them again on the occasion of his funeral.
Here's a fun alternative. Altaïr warns Maria, "Do not follow me." See how long you can follow her! Maria takes off running through the alleys of Jerusalem (mostly in the Poor District). You can chase her but you can't kill her. Watch out for those guard posts with drawn swords. I chased her for three and a half minutes once before losing her right in front of the cemetery.
ARSUF
The rest of the game is just one fight after another. There isn't much opportunity for style other than fighting with the Hidden Blade. Instead, I'll give you some shortcuts:
This last tip is more of a cheat than a shortcut. During the cutscene with Richard the Lionheart, get right up in Robert de Sable's face. When he says, "To arms, assassin." it's possible assassinate him immediately without facing his ten goons first. You want to be in "run away" mode with the Hidden Blade selected. Instead, you're automatically placed in fight mode with the Assassin's Sword armed (thanks Ubisoft!) Quickly pull the Left Trigger (L1 on PlayStation 3) to exit fight mode, arm the Hidden Blade, run up behind Robert de Sable and stab him in the back. That's an awful lot of work for your left hand. You can skip the first step by going into Options | Controls and turning on "Hold Trigger to Lock" before attempting this complicated maneuver.
I have personally confirmed that this cheat works. Here's the evidence. I must not be very good at it because it took me 3+ tries to get it done. What finally worked for me was tackling Robert and then switching to the Hidden Blade while he was getting back up to his feet. Your mileage may vary.