The Collaborative International Dictionary of English v.0.48:

Vanquisher \Van"quish*er\, n.
   One who, or that which, vanquishes. --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

vanquisher
    n 1: someone who is victorious by force of arms [syn:
         conqueror, vanquisher]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "vanquisher":
   champ, champion, conquering hero, conqueror, conquistador,
   defeater, easy winner, hero, master, pancratiast, runner-up,
   shoo-in, subduer, subjugator, sure winner, top dog, triumpher,
   victor, winner