Complex sentence: The artillery situation was only slowly improved by Gallwitz's plan to centralize counter-battery fire and use the reinforcements of aircraft to observe artillery fire, which had little effect on Allied air superiority but did eventually increase the accuracy and efficiency of bombardments.
Split into simpler sentences: The artillery situation was only slowly improved by the plan devised by Gallwitz to centralize counter-battery fire and use the reinforcements of aircraft to observe artillery fire. The extra aircraft had little effect on Allied air superiority but were able eventually to increase the accuracy and efficiency of bombardments.