After killing her about 20 times, I finally figured it out.
With any moving target, you have to lead the target. However, we're well-versed on doing it in a straight line. This, however, is moving along a curve and that's a lot harder to see very well. In this case, however, the game gives you the answer. If you look, the targets get positioned in pairs, eahc the same distance from the center of the wheel. If you hold the mouse over one of the targets under her arm and do NOT throw the knife, you'll see the other target under her arm will pass directly beneath your mouse. In other words, you're lined up on target. At that point, it's simply a matter of getting the timing right as you throw the knife.
Fun, eh?
Got 3120 on the very next game.
--Wag--