The Collaborative International Dictionary of English v.0.48:
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. [1913 Webster]WordNet (r) 3.0 (2006):
skirmisher n 1: someone who skirmishes (e.g., as a member of a scouting party)