Complex sentence: The advance of supporting troops was much easier to obstruct but it was more important to help the foremost infantry and if counter-battery fire was insufficient, the covering fire in front of the advance was more important and counter-battery groups should change target.
Split into simpler sentences: The advance of supporting troops was much easier to obstruct but it was more important to help the foremost infantry. If counter-battery fire was insufficient, the covering fire in front of the advance was more important and counter-battery groups should change target.