Equip defense against dark, death, and proant (best option is darkest night + death amulet). You should have both Julius (or whatever the protagonist's name is that you chose) and Serafina equip resurection in case Istaroth uses some other element attacks and kills your team member. Super Strength + Slayer Stance on Irito then cross slash or swan slash or golden slash.
Once you reached the 3rd stage (where Istaroth summons the 4 devil brother and those orb thing) you will get defeated. Then the option will pop up for your final move in which you chose layered awakening pronant. Then each of the sister will fight one of the devil brother, in which you go into [shift] -> Awakening Pronant then beat the them. Finally it's gonna be you vs Istaroth in which case you cast Awakening Pronant on yourself 4 times (after 3rd time an event occurs which your character need to to chose an item from your inventory, chose either Nami's or Cartana"s thoughts"). Then you release your final move (forgot name but it replaces your Archdemon's claws move) which leaves Istaroth at about 1/5 hp. Then just heal up and do another Super Strength + Slayer Stance on Irito then attack finally end the him for good.