Sundjata won the battle over sumanguru because sumanguru's magic was only ...

According to legends, the kings were magicians and their magic would decide who would be the victor. In the heat of the battle, King Sundiata angrily roared at King Sumanguru's soldiers. They ran for cover. While they scurried away, Sumanguru bellowed back, and the heads of 8 spirits appeared above his head; but King Sundiata's magic was much greater and had defeated the spirit. Sundiata then faced Sumanguru and shot an arrow at him. The arrow had only grazed Sumangurus shoulder, but it had drained all of his magical powers.