Argosax the Chaos is the final boss of Dante's Disc which is encountered in Mission 18.
NG+ Note: Same strategy as New Game and it can maintain Majin for phase 2.
If you don't have a Gold Orb from that point then it's not worth to go for Shin Majin set up. Because the probability of Griffon killing you is high. Hence, you want to either you get Majin set up after killing Griffon or die to get full DT gauge bar ready to perform Round Trip strategy. In Phase 1, you have to get hit by the Giant Laser 3 times. Avoid getting hit from the diffusion beam at all cost.
This is the order you have to follow when killing Argosax :
Nefasturris → Orangguerra → Large Devil Star → Phantom → Jokatgulm
→ Small Devil Star → Furiataurus → Small Devil Star Use it After Majin Ends → Griffon
This is how Phase 1 should usually go. Do not start attacking until Nefasturris head stays still, so the Whirlwind will do max damage
If you don't have Gold Orb, then this is how it should go from Furiataurus part. Maintain the Majin form until the very end and use Laevateinn against Griffon
There is no one way to do this fight. You as a player must know when it's a good time to enter Shin Majin form. You also need to be familiar with Despair's pattern as in what to answer to each situation :
Average Despair Fight
Ideal Despair Fight