Source : Webster's Revised Unabridged Dictionary (1913)

Antagonistic \An*tag`o*nis"tic\, Antagonistical
\An*tag`o*nis"tic*al\, a.
   Opposing in combat, combating; contending or acting against;
   as, antagonistic forces. -- {An*tag`o*nis"tic*al*ly}, adv.

         They were distinct, adverse, even antagonistic.
                                                  --Milman.

Source : WordNet®

antagonistically
     adv : in an antagonistic manner; "he behaves antagonistically
           toward his colleagues"