Sentence:  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 <WSD>guerrilla</WSD> 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.' 
Meaning of the word marked with <WSD> tags:  Relating to, using, or typical of guerrilla warfare, or its principles of small independent or non-official perpetrators.