Complex sentence: This was partially due to the fact that on many occasions, the G4M was used for low-altitude torpedo attacks on shipping in which their performance advantages were negated, and the G4M was frequently shot down by antiaircraft artillery fire, and even by small arms.
Split into simpler sentences: This was partially due to the fact that on many occasions, the G4M was used for low-altitude torpedo attacks on ships during which their performance advantages were negated. The G4M was frequently shot down by anti-aircraft artillery fire, and even by small arms.