The Collaborative International Dictionary of English v.0.48:

Cruelly \Cru"el*ly\, adv.
   1. In a cruel manner.
      [1913 Webster]

   2. Extremely; very. [Colloq.] --Spectator.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cruelly
    adv 1: excessively; "a cruelly bitter winter"
    2: with cruelty; "he treated his students cruelly"

Moby Thesaurus II by Grady Ward, 1.0:

87 Moby Thesaurus words for "cruelly":
   abominably, agonizingly, atrociously, awfully, baldly, balefully,
   barbarously, bestially, bitterly, blatantly, brashly, brutally,
   brutishly, confoundedly, damnably, deadly, deathly, deucedly,
   devilishly, diabolically, distressingly, dolorously, dreadfully,
   egregiously, excessively, excruciatingly, exorbitantly,
   extravagantly, ferally, ferociously, fiendishly, fiercely,
   flagrantly, frightfully, grievously, harshly, heartlessly,
   hellishly, horribly, improperly, inclemently, inexcusably,
   inexorably, infernally, inhumanely, inhumanly, inordinately,
   intolerably, lamentably, mercilessly, miserably, nakedly, openly,
   painfully, piteously, pitilessly, relentlessly, remorselessly,
   ruthlessly, sadly, savagely, sharkishly, shatteringly, shockingly,
   slaveringly, something awful, something fierce, sorely,
   staggeringly, terribly, torturously, truculently, unashamedly,
   unbearably, uncompassionately, unconscionably, unduly,
   unforgivingly, unhumanly, unmercifully, unpardonably,
   unremorsefully, unsympathetically, unyieldingly, viciously,
   woefully, wolfishly