The Secret Triwizard Challenge

Eyes are always a good thing to go for. Spiders probably don't have great hearing, and you know it was impaired by the lack of light earlier.
"Stupefy!" you yell, aiming for the spider's eyes. You get a direct hit, causing the spider to shriek in pain and claw at its eyes. While the spider is distracted, you manage exit the clearing into the next path, which the spider is too big to fit into.
Whew.