The Hidden Blade Assassin's Creed in Depth

Using the underground entrance as a way to turkey shoot (immobilize) the target.

Sequence 3 Memory 2 Confession

The underground entrance is on the south side of the building, near the back. On the wall is a strange symbol you can interact with. This same symbol is also by another underground entrance, the one by the front door. A large trap door is the opening inside Notre Dame.

Start by clearing the area of nearby guards who could spot you. Then get the target's attention and lead him to the stairs of the underground passage. I tried using dead bodies and cherry bombs to get him all the way underground, but they didn't work so good, so I resorted to using smoke bombs+tackle.

Once he's in there close the trap door. This isn't necessary to prevent him from moving because the AI of characters can't find a path to or from the underground entrance and the inside of the church.

To reach him from outside, go through the underground entrance nearby, the well. With Sivert being a sitting duck, you can sneak up and turn him into a human pinata with a melee weapon. A smoke bomb makes this more effective if you don't want him to try to fight back.