Source : Webster's Revised Unabridged Dictionary (1913)

Faultless \Fault"less\, a.
   Without fault; not defective or imperfect; free from blemish;
   free from incorrectness, vice, or offense; perfect; as, a
   faultless poem.

         Whoever thinks a faultless piece to see, Thinks what
         ne'er was, nor is, nor e'er shall be.    --Pope.

   Syn: Blameless; spotless; perfect. See {Blameless}. --
        {Fault"less*ly}, adv.-{Fault"less*ness}, n.

Source : WordNet®

faultlessly
     adv : without a fault; in a faultless manner; "he solved all the
           problems faultlessly"