Sephiroth is the side boss.He's harder than the last few bosses. He is hard when you fight him first but then once you get the hang of it , Sephiroth is easy. I defeated him on Expert. What makes him even more dangerous is that crazy attack called Send Heartless Angel (I heard clearly and does'nt say Sin Harvest.Also in FF 7 Sephiroth has this attack and he uses it.) which takes you done in one hit.Only an ability called Second Chance keeps you alive , but better keep moving... Right after Send Heartless Angel , he teleports and hits you , or goes up with that flame pillar attack.
Visit gamefaqs drifter. Look for the Sephiroth guide. lol. Gamespot (I think) had troubles beating him so they called technical support and asked.The guy replied back saying "Well are you level 100 yet?" lol I think it is gamespot...