Here is something I did in order to help for a speedrun of the game. It appears that the knives are the fastest method, but a hidden gun shot is faster than 2 knives, so use 14 knives + 1 hidden gun shot is the fastest. I don't use the crossbow (too low damage) or knives AFTER something else because it takes some time to put a weapon back in the belt.
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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.