I tried to break down how I would most likely tackle this one (after color studies).
Each outlined character would be a separate layer or layers
2.Gilderoy and the Photographer
3.Percy, the Grangers and Mrs Weasley
4.Lucius and Mr. Weasley (on separate layers)
5.Ginny and Malfoy
6.Ron and Harry and Hermione
7. Then Fred and George and the rest of the Witches
After that it would be touch-ups on everything and lighting effects.
I might jump around a bit between characters if I need a break. I would make sure my layers are organized. I like to slightly adjust the figures positions and/or limbs and it becomes a nightmare if the character is on 6 separate layers all over the place.
I do still like this sketch but I am not yet ready to paint it. I think the drawing still needs to be pushed more. :)