Agnus

Agnus is encountered as the level boss of Mission 9 then becomes part of the boss rush in Mission 19. This boss is arguably one of the hardest fight in the series due to the reasons below.

Players can easily auto-pilot with Nero that causes him to knock down in a wrong spot or lack of DT-management

Perfect cycle with Dante requires two Distorted Just Straights with the perfect-line up

Nero

Both fight starts the same way with CS'3 → DT → EX3-Calibur
Mission 7 - Conserve DT for his Gladius spawn. This allows you to min-max your damage while staying in DT longer however, you must turn off DT once he is stunned to save DT → CS'3 → EX3-Calibur for making him stay in the same spot after the first knock down.
Mission 19 - Basilisk allows you to use all of your DT-Gauge since you can Buster them to easily stun Agnus and a nice white orb drop.
The purpose of Slash → Aerial Combo is to ensure that you can react to his patterns and fix Nero's spot when he is being pushed back by inertia.

Dante

After inital the Straight into RI if your camera, movement or the sequence feels off then prepare to adapt and dodge Basilisk shots

Hardest fight in the game because of the two Full Just Frame Charges V1 | V2
As seen above, the movement and the camera line must be perfect for the Basilisk to not attack.
DRI usually make ups the loss if the first Straight 2 was not good.
Alternative second sequence is distorting last two hits of Combo A despite being risker, it usually salvages the situations better than a second Straight 2
When things go south make absoutely to stay on ground and spam Reb Combo A. This so your DRI won't whiff when you have opportunity to execute it.
Because Combo A requires six DT stock. Consider using the Back-up cycle that involves Overdrive kill against Basilisk.