Using a combination of the power of the Hakuba ritual inbued in the Vampire Killer itself, and Charlotte Aulin's knowledge of the "multilayer quantum-space barrier", Julius is able to use it to enter Dracula's mind, so he can find and destroy his connection with the Castle, in a reverse twist of Brauner's paintings in Portrait of Ruin.
The battle happens inside Dracula's psyche, a vast sea made of the blood of countless impaled humans with the castle looming over them. All this blood represents the souls of these humans. The scenario is the "will of the artist", but in this case the canvas is Dracula's mind, and the artist is Dracula himself.
In this dimension, Dracula's distorted spirit can be found, and is composed of a wolf, a bat and a demon, and in his hands, he holds the Crimson Stone, which he uses to draw his power from the Castle, and also is his weak spot. He's buried in heaps of the human souls he absorbed and enslaved along the centuries. These human souls, in turn, are submerged in the sea of blood.
It's all very symbolic, considering this happens inside Dracula's own personal dimension. The concept comes from "Whip's Memory" which has its own dimension inside its supposed psyche, with a full fledged Richter Belmont inside, and again, from Brauner's paintings and how Charlotte and Jonathan could enter it to destroy their core source of power.
The goal is to destroy the stone between his hands, which connects Dracula's soul to the Castle. The Castle in the background represents the connection Dracula has with it inside his spirit/mind.
When Julius approaches, he's able to climb the small hill of souls and hit the stone. After hitting enough times, Dracula reacts.
Dracula has some attacks: The wolf head is able to vomit spheres made of blood to hit Julius when he's near, the bat is able to flap the wings very fast so he can send Julius flying far from the Stone, and the demon head's forehead eye can shoot spheres of energy towards Julius' position that explode when making contact. The Stone itself can shoot red lighting bolts towards Julius, and can create waves of bats made of energy that hit multiple times (Akin to Dracula's "Bat Moon" attack on Order of Ecclesia). The Stone can also heal itself by absorbing Red Zombies (Which are human souls summoned to attack Julius).
Everytime Julius attacks the Stone, the Castle in the background can be seen "phazing" in and out of reality.
Near its destruction, the Stone summons a doppelganger of Walter Bernhard made of human souls, and shields itself, so only after destroying "Walter" it can be attacked again.
When the Stone is destroyed, the Castle in the background suddenly disappears, symbolizing the broken connection between Dracula's Spirit and the Castle, and Julius is kicked back to reality.
Also, this is why Julius loses his memory: This is the price to pay for entering Dracula's own psyche to fight him, to lose a little from his own mind.
I even imagine his musical theme to be "Arch Enemy" from Harmony of Dissonance, remixed to use a lot of Kingdom Hearts' style violins.
And yeah, I made a mockup representation of it:
That's it dudes! I REALLY loved this challenge, as I always wanted to create a Castlevania final boss! Making everything took me a whooping 10-days time.