"Now, yer mum an' dad were as good a witch an' wizard as I ever knew."
(Harry Potter and the Sorcerer's Stone)
This comparative construction, I suppose, has the omission of “counterpart of comparative phrase normally omitted (CGEL,p.1108)”. From Wendikidd’s comment, the complement of as before omission would be ‘any of those I ever knew’ or ‘those I ever knew’ (those: good witches and wizards). Are both accepted or one of them?