Final boss mechanic

[VillageIdiot]

I seen a video where someone killed the final boss on POTD upscaled without phase transition, i can only do this on normal, anyone have any idea how to do this?

 

It seems every time i even do slightest big damage he just goes invulnerable and transforms. 

[VillageIdiot]

My party composition is:

 

Eder: Fighter

Maia: Ranger

Tekehu: Chanter

Xoti: Priest

Me: Wizard

 

all pure class.