Easiest Way To Defeat Fortibras:

You need at least one Soul Absorber for this trick. Equip your Lightning Sword and slash Fortibras 2 times when the fight starts. Wait for him to attack. As soon as he starts his attack use your magic on him with the Lightning Sword. Repeat the process until your magic ends. Now use your other weapon’s magic on him until you are out of magic. Once you are out of magic for all your weapons equip your Lightning Sword again. Then without exiting the menu use the Soul Absorber (make sure you are standing near him, so that he is in range of the Soul Absorber). Exit the sub menu and press the "o" button to absorb the souls. This will replenish your magic again. Now use your lightning magic again and he'll be on his way to hell.