The Collaborative International Dictionary of English v.0.48:

Guileful \Guile"ful\, a.
   Full of guile; characterized by cunning, deceit, or
   treachery; guilty. -- Guile"ful*ly, adv. --
   Guile"ful*ness, n.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "guilefully":
   artfully, astutely, cagily, cannily, cleverly, cunningly,
   deceitfully, foxily, fraudulently, furtively, indirectly,
   insidiously, knowingly, on the sly, shiftily, shrewdly, slick,
   slyly, smoothly, subtilely, subtlely, surreptitiously,
   treacherously, trickily, underhandedly, warily, wilily