The Hidden Blade Assassin's Creed in Depth

This mission is similar to the parachute air assassination as shown by Arrrogance:
http://www.thehiddenblade.com/cardinal-sin-horse-exploit-and-parachute-k...
The parachute eagle strike is a lot easier than it looks. I estimate that I have about a 50% success rate on the eagle strike (maybe better).

A brief tour of the assassination site
Familiarize yourself with the target's starting location and the structures around the target. You'll need to be able to spot this location from the top of Castel Sant'Angelo. Two key landmarks are the fountain and the chimney on the roof. The target starts near the corner of the building. He will start moving when he has been identified, so you need to avoid getting too close to him or using eagle vision.

Parachute Eagle Strike
You need to reach the top of Castel Sant'Angelo without being detected. Here is a relatively fast and reliable way. Some notes:

3:06 Aim the left control stick in the 1 o'clock position while running up the wall. Thanks to massmurder, who reported this shortcut, and thanks to ThreeLetterSyndrom for making a video of it.

3:25 Make sure Ezio's hands are in the same position as what is shown in the video. Aim the left control stick in the 1 o'clock position while performing the climb leap. If you have trouble with this shortcut, you can just go up and around instead.

5:00 Aim the initial jump directly toward the target by looking over Ezio's head, pressing directly up with the left stick, and tapping the legs button. Immediately deploy the parachute. You can fine tune the parachute path as you approach the target.

5:39 Use visual cues to precisely time the drop from the parachute. I specifically look for the chimney on the roof to approach the bottom of the screen. I drop down just before the bottom edge of the chimney touches the edge of the screen.

5:42 Look down as you fall. When the target is highlighted, lock onto him, grab the ledge, and then air assassinate. If you miss catching the ledge, Ezio will die.

Additional Takes
Here are a couple more takes that I did for the parachute eagle strike. The first one didn't require a lock since I caught the ledge so close to the target. In the second clip, I accidentally bumped the call horse button as my thumb went from catch to air assassinate. In both of these clips, the eagle strike takes just a little longer due to delays in targeting that are common with many eagle strikes.