3rd task

Here's a response three years late... LOL. Well, this could help someone else out who happens to be looking here.

Anyway, you stand near the wall, or the hedge, and wait for one or both of them to charge you. Move out of the way before they hit you, and when they hit the wall, they will be dazed momentarily. That's when you get behind them and jinx them. You have to do it a couple times, and it might take a bit of practice, but you'll get it.

Or you could just read this fellow's advice: http://www.gamesover.com/HP4/Level9_1.htm