guerrilla

partisaani, sissi puhekieltä A non-official war carried out by small independent groups; a guerrilla war.
A soldier in a small independent group, fighting against the government or regular forces by surprise raids.
puhekieltä Relating to, using, or typical of guerrilla warfare, or its principles of small independent or non-official perpetrators.
1908, George Devereux Oswell, Sketches of Rulers of India, volume I, Chapter VII, page 127:

Wherever Nicholson was most wanted, there he was sure to be found. What his life was at this time may be seen from a letter he wrote to his mother: 'I am leading a very guerrilla sort of life with seven hundred horse and foot raised among the people of the country. The chieftain who is in rebellion has eight regular regiments and sixteen guns, so that I am unable to meet them openly in the field.'

On the other hand, after the fall of Feng Ling Tu, the operations of Central Shansi, and Suiyuan, troops were more guerrilla than orthodox in nature.

The Slovak uprising in the latter days was not guerrilla in character, which, incidentally, may have been one of the reasons for its failure.

We took a very guerrilla approach to marketing Goin' Hollywood, in keeping with the irreverent tone of the game.