From what I've gathered by reading the blog entry this is a collaboration of several artists.

Flandre and Koakuma are by domazo no yuuki (http://www.pixiv.net/member.php?id=43721); Patchouli by fuukadia (http://www.pixiv.net/member.php?id=2991); the lineart of Meiling by sunaipu (http://www.pixiv.net/member.php?id=2897); colouring of Meiling and Sakuya are by yatosaki haru (already tagged). I don't know if the one who drew Remilia is mentioned later on; yatosaki doesn't know their name it seems.