Exploit the A.I.:
When playing as Dante, in the battle against Argosax the Chaos you can actually exploit the reach of the boss and its A.I.. Walk to the side where the tentacle is and where the furitaurius is stand between them
just out of the reach of the tentacle and the furitaur's fire, and you can sit there and shoot them up for devil trigger all day and not get hit because the tentacle is too short and the fire cant reach that far.
Infinite Red Spheres:
Just do this: in a mission, find some Red Spheres, then press START and restart the mission, you'll have the spheres you've found in the mission that...you restarted! It works correctly in Nefasturris mission.
When you beat Nefascapitis, immediately press START and restart, you'll be able to collect infinite red spheres.