Hot Foot Assassination: The Arrow
The Arrow is perhaps the easiest assassination target in the entire Assassin's Creed series. The Firing The Arrow memory begins with Eivor onstage and standing directly behind the target. All one needs to do is take three steps forward and stab him in the back. Furthermore, Eivor can do practically anything without raising suspicion among the target, the other archers gathered onstage, or onlookers. In other words, it's a perfect opportunity for a creative assassination!
I suggest climbing the large arch behind the stage. From the top you may air assassinate The Arrow. It's pretty cool but I wanted to try something other than an air assassination for a change.
HOT FOOT
There's a classic prank among baseball players called the hot foot which involves placing a flame near the victim's foot. We can do something like that in Valhalla. First we need to find one of those flaming oil jars. They aren't very common in Lunden and most are located in heavily guarded areas. The one I grab in the video has the virtues of being unguarded and easily accessible via a road running west from the stage. There may be closer oil jars - I don't know.
Carry the oil jar back to the stage and place it at The Arrow's feet. You can put it right in front of him. He won't care. Eivor is golden. I prefer to put it directly behind The Arrow so that the blast will blow him off the stage.
Now retreat to a safe distance with a clear shot of the oil jar. When The Arrow introduces his "new elite archery force" that's your cue to loose an arrow and demonstrate what a good shot you are.
This is where the game makes you stand over the body and hold 
for 5 seconds to "Confirm Kill". As if a burning corpse isn't dead enough, now we have to stab it too. This sucks because the escape I had planned begins from the same place where I fired the arrow. Now I have to climb up there again after the assassination.
HIGH WIRE ESCAPE
Escaping the area is also really easy. Run in any direction except north and you should be fine. Stowe is waiting for Eivor to the south so escapes in that direction are preferred.
I demonstrate a ridiculous escape based on The Floor is Lava game. It's possible to get from the assassination site to Stowe without touching the ground. The first part zig zags over the audience as arrows whiz by.