Sephiroth is a tough boss but, with some strategy and the right skilsl you can defeat him. Okay first, equip all of your strongest gear and a few healing items. Next equip these abilities: Retealing slash, finish plus, block, once more, second chance, aerial dodge, glide, high jump (lv.2 will do),sliding dodge, these abilites are important. the battle will begin with a reaction command, after that attack him with a combo and try to do it twice. when he knocks you in the air use retealing slash so you will hit him and stop his combo. if he is close to you use block twice so you won't lose any health. After a while he will summon dark balls, attack them or use reflect. If he teleports, rise into the air, and he is glowing red, use the high jump, then aerial dodge close to him an attack(if you don't stop him he will drain your health and magic). When he creates giant flames use glide to stay away from it. With more helath down he will turn complete red. He will do the same moves but, more powerful he will also summon commets, just keep attacking him and you will defeat him.