The Hidden Blade Assassin's Creed in Depth

http://www.youtube.com/watch?v=6XHnGjPn6Jo

Since thrown heavy weapons cause instant death, the developers intentionally made Cesare impervious to this type of attack in combat. Thrown heavy weapons bounce off of him and then fall to the ground. You can circumvent this when you are not in open combat with Cesare, which happens automatically if you are more than 15 meters away from him. Thankfully, there's just enough room on the castle wall to pull this off.

Blitz Assassination
A prerequisite for this mission is to have a heavy weapon in your sheath. Start by luring Cesare to one end of the battle area. Sprint past him to the other end. When you are more than 15 meters away (but fewer than 20 meters), turn around and throw a heavy weapon before the other guards get in the way. This blow instantly kills Cesare, but due to scripting of the game, you need to go through some formalities to complete the mission...

After mashing the attack button to chip off some armor, you'll find a "dead" Cesare standing with the heavy weapon lodged in his skull. You need to injure him some more in order to continue chipping his armor away. The problem is that many weapons do not have any effect on him now that he is "dead"... Swords, for example, pass right through him. I found that the fastest way to progress the game is to use the hidden gun, which needs to be aimed manually since you can't lock on to dead people. You also need to clear the battlefield to use the gun efficiently since the game automatically aims it at any nearby guards.

1:38 After chipping off some armor, the game disables the hidden gun for a few seconds. I suspect that the developers did this to prevent you from wasting ammo in case you got carried away with mashing the attack button.

Double Bearded Axe
Bastard Sword and Bearded Axe
After Cesare's armor has been all chipped away, Brutes will start appearing in battle. You can use one of their heavy weapons to deal the finishing blow to Cesare. You will need to aim manually since Cesare is already dead. Yes, his head can hold more than 1 heavy weapon at the same time.

Recurring Migraines
You can "pick up" the heavy weapon from Cesare's head while he is standing, and then re-use the heavy weapon. (Incidentally, you can also loot him and pick up his dead body while he is standing, too.)

Non-story Assassination
In the previous clips, Cesare is left as a standing dead man after getting a heavy weapon thrown at him. You can make him look like a "proper" dead man quite easily. There are 3 main ways, but I show only one of them in the video.

1) Run to him and knock him down.
2) Let the guards knock him down as they run to you in battle.
3) Walk up to Cesare and press free hand to pick up his (standing) body, and then drop his body.

Also shown in the video is a demonstration that you can loot Cesare (who carrys nothing), pick up your thrown weapon, pick up his body from the ground, and toss his body. You can't toss it over the castle wall because the game puts up an invisible barrier. UPDATE: Greyz174 on Youtube says that you can throw his body over the wall if you stand on "that box on the edge of the castle."

Now that Cesare is laying on the ground, you won't be able to injure him any further. With no way to progress the game, you might as well let the guards kill you so that you can restart from the last checkpoint.