Guerrilla

a member of a band of irregular soldiers that uses , harassing the enemy by surprise raids, sabotaging communication and supply lines, etc.
pertaining to such fighters or their technique of warfare:
guerrilla strongholds; guerrilla tactics.
Contemporary Examples

Almost as soon as the last Southern king fled into exile, a guerrilla movement took form among the peasants of Sicily.
David’s Book Club: Who Ruined Southern Italy? David Frum July 30, 2012

Known for its guerrilla journalism, Vice Media has linked up with corporate giants in an investment deal.
A Hipster Media Company Goes Mainstream Nicole LaPorte April 4, 2011

This, from a hardened revolutionary, from a man who trained in guerrilla warfare and plotted to overthrow the apartheid regime?
Nelson Mandela’s Revelatory Diaries James Zug October 15, 2010

This time around, having weathered 2008, the army is fully prepared and savvy in guerrilla warfare.
Time for Palin to Embrace Her Gender Amy Siskind April 3, 2010

The company decided to use what Okochi calls “guerrilla marketing.”
In Japan, Zima Haz No Zexual Preference Jake Adelstein, Angela Erika Kubo September 12, 2014

Historical Examples

You took the part of Gendron, when you knew he was nothing but a guerrilla and a horse-thief.
Young Captain Jack Horatio Alger and Arthur M. Winfield

The moon was behind us, for the guerrilla was on the western side of the mesa.
The War Trail Mayne Reid

The war which followed resembled the guerrilla conflicts of Kansas, with the addition of the Indian element.
The Last American Frontier Frederic L. (Frederic Logan) Paxson

It was not the guerrilla that was uttering that cry; it was the yell of the Indian warrior.
The War Trail Mayne Reid

guerrilla war (always successful, as history shows) directly infringes that rule.
War and Peace Leo Tolstoy

noun

a member of an irregular usually politically motivated armed force that combats stronger regular forces, such as the army or police
(as modifier): guerrilla warfare

a form of vegetative spread in which the advance is from several individual rhizomes or stolons growing rapidly away from the centre, as in some clovers Compare phalanx
n.

“fighter in an irregular, independent armed force,” 1809, from Spanish guerrilla “body of skirmishers, skirmishing warfare,” literally “little war,” diminutive of guerra “war,” from a Germanic source (cf. Old High German werra “strife, conflict, war;” see war). Figurative use by 1861. As an adjective from 1811. Acquired by English during the Peninsular War (1808-1814); purists failed in their attempt to keep this word restricted to “irregular warfare” and prevent it taking on the sense properly belonging to guerrillero “guerrilla fighter.”