I am stuck at the three headed snake. How do you defeat it?

When you see a snake open it's mouth on the top screen, get away from the area it'll strike. There should be a couple piles of rocks. Levitate a rock and when the snake's head hits the ground, throw the rock at it. It'll be knocked out for a few seconds, and you can use Stupefy to hurt it during that time (Ron will help with that). Once you defeat the monster, spiders will come towards you from the lake. This part is really easy, just use Stupefy on them. After that, the snake will appear again. But this time, two heads will strike at once. I haven't gotten past this part, but I imagine it's the same thing, just find an area that you won't get knocked back before you can throw a rock.