Very easy way to SSS Agnus with Dante. (And I wish I knew this before succeeding with Nero, would save me a lot of trouble.)
I did this on LDK mode, though it should be able to be done on SOS & definitely on DMD. I recommend do it on DMD as my last attack just gave me SSS.
Have super Dante, switch to gunslinger style, activate devil trigger, have Pandora equipped, triple jump into air and press B bottom (default controls) and keep pressing till he's dead. If he get of a hit off, just die and choose continue so u can restart the boss fight.
Just to save everyone time this will work for Berial, Echidna, but will not work for Bael (the giant frog), maybe on DMD MODE.