The achievement is pretty simple, just unlock all of the Witch characters in the game. This can be quite a lengthy achievement to get.
List of all the characters;
Hermione Granger
Hermione (Ball Gown)
Hermione (Gringotts)
Hermione (Grey Coat)
Hermione (Ministry)
Hermione (Mafalda)
Hermione (Cardigan)
Hermione (Red Dress)
Hermione (Jumper)
Hermione (Scarf)
Alecto Carrow
Bathilda (Snake)
Bellatrix Lestrange
Bellatrix (Azkaban)
Charity Burbage
Cho Chang
Cho (Winter)
Professor Umbridge
Umbridge (Wizengamot)
Emmeline Vance
Fleur Delacour
Ginny Weasley
Ginny (Dress)
Ginny (Pajamas)
Professor Grubbly-Plank
Hannah Abbott
Hestia Jones
Madam Irma Pince
Katie Bell
Lavender Brown
Lily Potter
Lily (Young Casual)
Luna Lovegood
Luna (Blue Jumper)
Luna (Overalls)
Luna (Pink Dress)
Luna (Purple Coat)
Luna (Yellow Dress)
Madam Hooch
Madam Pomfrey
Madame Malkin
Madam Rosmerta
Mafalda Hopkirk
Marietta Edgecombe
Mary Cattermole
Molly Weasley
Molly (Apron)
Mrs. Figg
Narcissas Malfoy
Professor McGonagall
McGonagall (Black)
McGonagall (Pajamas)
Nymphdora Tonks
Tonks (Pink Coat)
Padma Patil
Parvati Patil
Penelope Clearwater
Professor Sprout
Professor Trelawny
Rita Skeeter
Susan Bones
Mrs. Black
Good luck :)