past master 
noun
1A person who is particularly skilled at a specified activity or art: he’s a past master at keeping his whereabouts secret...
- He is a past master at sending out novices to win big handicaps and I've been following the latter named six-year-old's progress since riding him in a couple of uphill canters on the trainer's Gloucestershire gallops in March last year.
- That's micro-managing cost controls, something that he is a past master at, as the cabin attendants (cabin crew) are finding out.
- But then, he is a past master at having it both ways.
Synonyms
expert, virtuoso, master, wizard, genius, artist, old hand, adept, professional, doyen, veteran, maestro, connoisseur, authority, grandmaster, master hand, skilled person
informal ace, buff, pro, star, whizz, hotshot, ninja
British informal dab hand
North American informal maven, crackerjack
2A person who has held the position of master in an organization: he was a past master of the City Company of Grocers