Perhaps: a partisan. rare.

A volunteer or freelance soldier. Obsolete.

Military. A member of a small body of light or irregular troops operating independently and engaging in surprise attacks, etc.; a guerrilla. Also…

An irregular combatant or skirmisher, accustomed to fight in the bush; one who fires from among the bushes.

One engaged in such warfare.

= guerrilla, n. 2.

spec. In wider sense: One who resembles a ‘filibuster’ (sense 2a or 2b) in his actions; now esp. one who engages in unauthorized and irregular…

A person (esp. a guerrilla) who strives or fights in support of Islam; spec. an Islamic fundamentalist guerrilla. Also (in quot. 1921) irregular: =…

a terrorist or irregular combatant operating in an urban area.