When Nobunaga absorbs a magic power orb from a treasure chest, he will most likely absorb it. If he does, do not try to absorb it. The orb will go towards him, and not you. Instead, after he has the orb, hit him by holding R1 and pressing Triangle. His sword should fly up, and the orb should float away from him. While he is stumbling from the hit, quickly absorb the orb then use the magic attack by holding R1 and pressing Triangle. Try repeating this process. Also, you can charge up your weapon by holding R1 and Square for four to five seconds.