。
He was ambushed by a guerrilla squad.
Literal
He [topic-は] guerrilla [possessive-の] one-squad [to-に] ambushed [passive-された].
ゲリラ is the loan 'guerrilla.' 一隊 (いったい, 'one squad/party') is the counter for military units. 要撃された (ようげきされた, 'was ambushed/intercepted') uses a military term in passive form. A concise military narrative sentence.