The Secret Triwizard Challenge

It's got eight dang legs, you should be able to hit at least one of them, no sweat. You whisper the incantation for the Jelly-Legs Jinx and aim at the giant spider.
Immediately, the spider starts wobbling. It can still walk, but you can see how much effort it takes for the spider to get anywhere. It doesn't seem to know where the spell came from.
Now that the spider is mostly out of the picture, you need to proceed into the clearing. How do you proceed?