Complex sentence: The covering fire was so effective that the Ottoman defenders had extreme difficulty in taking aim and firing their rifles and machine guns, and so it became possible for the attacking forces to cover the last of smooth grassy slope in two rushes.
Split into simpler sentences: The covering fire was so effective that the Ottoman defenders had extreme difficulty aiming and firing their rifles and machine guns. It then became possible for the attacking forces to cover the last of smooth grassy slope in two rushes.