Source : Webster's Revised Unabridged Dictionary (1913)

Agonizingly \Ag"o*ni`zing*ly\, adv.
   With extreme anguish or desperate struggles.

Source : WordNet®

agonizingly
     adv : in a very painful manner; "the progress was agonizingly
           slow" [syn: {excruciatingly}, {torturously}]