Firecracker Assassination: The Leech
You will find The Leech roaming the ground floor of the two story basilica in northeast Lunden. For a good stealth assassination enter through the open window on the second story and make your way over the study at the east end. Wait until The Leech arrives and hit her with an air assassination.
If at any time The Leech becomes spooked she will flee out the south door to the center of the courtyard. In doing so she passes a natural arrangement of four oil jars. With two more oil jars and a little imagination this becomes a perfect opportunity for a firecracker assassination.
The game mechanics make it difficult to move more than one oil jar a great distance. Whenever you place an oil jar and then move out of its sightline, that oil jar will disappear and respawn at its original location. Successfully moving two or more oil jars involves shuttling them 50 feet at a time and never around a blind corner. It usually isn't worth the effort.
PREPARATION
There are two preliminary steps not shown in the video. First, pre-assassinate all the guards in the courtyard being careful not to aggravate The Leech. Be particularly careful with the guard standing at the door of the basilica (the one through which The Leech flees). If she sees a dead body lying there it may spook her. So either whistle him away from the door or assassinate him while she's not looking and quickly move the body away.
Second, go find another oil jar and bring it to the courtyard. I grabbed the one right in the middle of the Temple of Mithras which is just south of the courtyard. The oil jar is on the ground below an archer on a high platform. You'll have to kill a guard or two in order to carry it out.
ARRANGING FIRECRACKERS
When compared to Assassin's Creed III, the oil jars in Valhalla have a very small blast radius of about four feet. This causes them to explode in rapid fire succession much like real firecrackers. Arrange the oil jars however you like maintaining a spacing of about four feet. Ensure at least one oil jar is on the north-south sidewalk that bisects the courtyard. Further down that sidewalk on the right you'll find another oil jar for your chain.
AGGRAVATE LEECH
Now it's time to roust The Leech into leaving the building. The hard part is doing it in such a way that gives you time to exit first and get into position. I chose the standard air assassination route (described above) waiting for The Leech to enter the study. Firing an arrow will get her attention whether it hits her or not. Drop down and sprint out the door. The ideal position allows you to take aim at the first firecracker and simultaneously keep an eye on The Leech's approach. When she's close enough release the arrow and spring the trap.
Very creative!