Possessed Arius is the final boss of Lucia's Disc which is encountered in Mission 13.
This fight has two phases and are completely trivialized by Aerial Heart + Flame Heart.
Transforms into Arius-Argosax in phase 2.
Much similar to Mundus, this boss is essentially an auto-scroller.
NG+ Note: Same strategy as New Game.
Arius-Argosax only gets stunned, if an attack hits the face.
There is a rare attack pattern that Possessed Arius performs which is the Rubble Throw. This object is breakable and has a value of 80 HP
It's very important to have Cutlaseer upgraded as Lv3 and you at least afford 2 Large Devil Star. If you are unable to afford the third Purple Orb then you'd want to start the fight with Cranky Bombs for max DT Build.
In phase 1, try to avoid hitting arius from behind a lot, since you are going to miss a lot of damage
Phase 2 is self-explanatory as long as you hit the face in a sweet spot, you are going to deal massive damage thanks to Flame Heart Glitch