Starting from the checkpoint when you reach the Palazzo Ducale, head left to quickly reach the scene location without being seen. Hang above the scene starter while getting a guard's attention. Make sure he walks onto the ledge leading into the room. Drop down to the corner of the awning (far enough away from the center that you don't start the scene but not so much that you stumble off the awning) in the way shown in the video. Immediately throw a smoke bomb to avoid detection. Now throw a knife at the guard to move him into the room. If he isn't positioned correctly, the guard won't fall into the room, and you will have to start over. After that climb up the wall and air assassinate the guard. You will jump over the scene starter without activating the scene. Now that you're in the room you can assassinate Carlo before the scene. One of the only ways to kill him at this point is with a melee weapon. There's also another glitch: Carry his body when you activate the scene (gotta love how the Doge turns his head to look at me carrying the body) . Carlo won't appear in the scene. However he "comes back to life" and runs away as usual. The reanimation happens whether you carry Carlo or not. Since Carlo is already dead and immune to any kind of attacks there is no way to progress through the game without quitting.
A Few Other Things
Just a couple more stuff worth mentioning when trying to pre kill Carlo. Air assassinations fail; grabbing and attacks with fists as well as most weapons don't affect him; you can steal from him, but it doesn't really do anything; he can be air tackled, but teleports back into the chair; the hidden blade can't be used to kill him unless you tackled him to the ground. And it also shows how Carlo is still in the scene if you're not carrying him when you start the scene.
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The Carlo Grimaldi assassination is somewhat scripted and linear, so I put as much creativity in this video as possible.
Ezio's exploits in the San Marco district, Sequence 8.
The goal of this mission is to kill every guard in the Doge's Palace and Carlo Grimaldi without being seen at all. That means no yellow bars over your opponents.
Most of my videos start at the checkpoint immediately before the assassination mission. This missions starts earlier - at the checkpoint when you land on the roof.