I didn't have much trouble on this fight. I found Shiva much harder. Anyway the party I used to defeat the Magus sisters was Warrior, Dark Knight and a White Mage. My Dark Knight kept using Black Sky because I didn't want to risk using Darkness to find that my Dark Knight died. My Warrior attacked and occasionally used Excalibur. My White Mage started by using protect and shell then cast Pray on every turn maybe casting a Cura if anyone needed it. Overall one the easiest boss fights in the game for me.