Source : Webster's Revised Unabridged Dictionary (1913)

Skirmisher \Skir"mish*er\, n.
   One who skirmishes. Specifically: pl. (Mil.) Soldiers
   deployed in loose order, to cover the front or flanks of an
   advancing army or a marching column.

Source : WordNet®

skirmisher
     n : someone who skirmishes (e.g., as a member of a scouting
         party)