Ronald Bilius Weasley

You feel that the best way to do this is under your best friend's nose though he could risk the entire operation.

Why? The only person who could hog food like Crabbe and Goyle is the shitlord himself. You start preparing for Ron's Arrival with a simple potion of bleaching powder at hand. He would arrive at Ms. Muriel's place, then back at Hermione's.

A few days later, you send a Parcel under Muriel's name and placed an Extendable Wireless Ears to overhear him (under a Disillusionment Spell). Sure enough in a few moments, Ronald's farts could be heard even without the ears. The toilet's clogged up and then Hermione took him to St. Mungo's Hospital as he faced from immense Diarrhea. As far as the wizarding world is concerned, it will still take a few months to recover. Hermione wouldn't even try to clean it as it stinks and she had a fair share when she met the troll in your first year.

Mrs. Granger then began calling out to nearby plumbing services, to which, thanks to Mrs. Figg's milk productions have been directed to you. You take the Plumber's disguise and begin proceeding to her place.