Agni & Rudra are first encountered as the level boss of Mission 5 then becomes part of the boss rush in Mission 18. The general idea of this fight is to damage them equally and kill them both at the same time to avoid either of them entering the superpowered fusion mode.
Both characters have the same approach in M5. Difference is how they clash. Dante can do it with either Cerb Revolver or Release.
In case of Gold Release happening, you can time your Stinger to knock the weapon away still. However, it's not always guaranteed due to the variance in positioning.
In theory, it can be faster to start with release because it immediately triggers A&R attacks, but the positon will be messed up where you won't be able to SC them simultaneously.
Vergil has to time his Yam Aerial Rave twice then Combo A FE.
Mission 18 is significantly faster than inital encounter.
First encounter fight with Dante. The quick-kill usually depends on your Shotgun cancelling speed.
First encounter fight with Vergil. Managing the DT and FE timing is the key factor of this fight.
Make sure to walk one step forward before releasing.
DTE is the key decider for this fight so make sure to use devil star before entering this room.