I am Setsuna | Dark Samsara

Dark Samsara is the 'big boss' of I am Setsuna. To defeat him, you have two strategies: [1] Defeat his four parts first. Or, [2] Just finish him off!

I prefer you to do the #2 because if you're hunting for achievement, you can obtain a hidden achievement: Total Destruction trophy, by defeating Dark Samsara in his true form. It may be a little bit difficult since Dark Samsara can regenerate his HP every round (lesser HP regeneration if you kill all of his parts). But if you make a preparation, then it will be an easy task.

As my preparations, I equipped my characters with attack/defense boost, recover HP/MP when attack, and some other buff-functioned spritnites. This is the list of my characters for this battle, for your reference:

ENDIR
Support: Resonance, Physical Pride, All-Seeing Eye, Reginleif, Lifeforce Logic, Commanding Wave, Fatus, Megalith, Vinculum, Gratia. 
Command: Battlecry*, Aura. 
Talisman: Daemon Impact 

NIDR
Support: Commanding Wave, Physical Pride, All-Seeing Eye, Reginleif, Ultimate Truth, Guiding Power, Lifeforce Logic, Resonance, Megalith, Enigma, Destined Cycle. 
Command: Rebellion*. 
Talisman: Brave Will 

KIR
Support: Commanding Wave, Wise Testimony, Lifeforce Logic, All-Seeing Eye, Reginleif, Sigtyr, Fated Memory, Soul Rage, Megalith, Enigma, Destined Cycle. 
Command: Meltdown* 
Talisman: Sacred Heart 

* Battlecry + Rebellion + Meltdown = Armageddon.
This combo attack can give debuff effect to Dark Samsara, and buff the characters at the same time. The buffs/debuffs are affecting the attack, defense, speed, and evasion for both party and the boss.

With the debuff, after a few attacks you get from Dark Samsara, you can unleash the Rebellion with high damage.

After Dark Samsara is defeated (but not yet...), there will be a long conversation between the characters and you will learn the truth about a 'youth'. After you go into the light, you will be back into the past and you're landed in Nive Harbor. Exit the harbor and head to the village. You will be welcomed by Raishin. Later, he tells you about a youth he saw at Falling Snow Monument. Go there by walking through Dazzshire Woods.

You will meet Youth (obtain another hidden trophy: From Beyond Oblivion). Then, there will be a final battle, but too easy to handle. But first, make sure you listen to ALL of his lines, or you will not get the final hidden trophy: The Light at the End. He uses Inner Darkness attack that only give you scratches. After you listen to him, finish him off. It will only take 1-hit kill.

And now, make your own finale and enjoy the ending!