Easiest: Safer Sephiroth (Beaten by Cloud's Omnislash equipped with last weapon.)
Ultimecia (Beaten by Sqaull's Lionheart equipped with Lionheart.)
Hard: UnderWater Weapon (Beaten by Cloud's Omnislash equipped with last weapon 20 times)
Ruby Weapon (Beaten by Cloud. Used Knights of Round 12 times)
Necron (Beaten by Zidane equipped ???Orcalic??? Used Sacrifice)