The Collaborative International Dictionary of English v.0.48:

Masterfully \Mas"ter*ful*ly\, adv.
   In a masterful manner; imperiously.
   [1913 Webster]

         A lawless and rebellious man who held lands masterfully
         and in high contempt of the royal authority.
                                                  --Macaulay.
   [1913 Webster]

WordNet (r) 3.0 (2006):

masterfully
    adv 1: in a skillfully masterful manner; with the skill of a
           master; "the young boy played the violin sonata
           masterfully"

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "masterfully":
   ably, adeptly, adroitly, agilely, aptly, arbitrarily, artfully,
   artistically, autocratically, brilliantly, capably, cleverly,
   competently, cunningly, deftly, despotically, dexterously,
   dextrously, dictatorially, efficiently, excellently, expertly,
   featly, handily, high-handedly, imperiously, ingeniously,
   magisterially, neatly, nimbly, overbearingly, peremptorily,
   proficiently, resourcefully, skillfully, spryly, superbly,
   tyrannically, well, with consummate skill, with finesse,
   with genius, with skill