When in the Chamber of Secrets part where there are several Lumos and Spongify Jump combinations, use Lumos on the first gargoyle then use Spongify to jump to the now visible ledge. Walk around the corner where you will see a ramp. Stand at where the ramp meets the floor and wait. When the Doxy reaches you, it will dive to bite you but will get stuck on the ramp, making it an easy target. Note: At least one Doxy must be alive for this to work.