I struggled mightily with this one for a few goes until I got a strategy that worked. Got a 5-star with plenty to spare.
Initial three that you fight - just Stupefy them till dead
Next 5 (3 + 2 more spawn) around the first whomping willow - Confringo them
For the three that are behind the tree - stand to the right of the tree and Cunfund one to kill at least one of the others - they're a little hard to hit from there with confringo unless you go straight down the path but then there's a higher chance of dying. Just kill the remainder with Confringo.
For all remaining - hit them with Confringo. The b***h of this one is that there are so many guys who can knock you down and then you're usually screwed. Confringo one hit kills most of them so you can hit them before they knock you down. Do a nice steady pace, and once you're down at the bottom, try and get them on their own and then confringo them.