Has everyone forgotten? If you remember, AoS mentions a war against Dracula in 1999 (Demon Castle War). I think the final battle should be in 3D and should start out with the teleport/fireball routine. Then after he is worn down a little bit, he says that he was just testing you and changes strategy like in OoE. Then after you deplete his life gauge, he summons a bunch of enemies to distract you while he takes the time to tap into the true depths of his power to a point in which he never has before. At which point, his physical body vanishes and his essence merges with the castle so that instead of just turning into a demon, you have to fight the entire castle. This will be done in sections due to the fact that his essence can only occupy one area at a time. Once an area is cleared, the Hakuba (spelling?) priest casts a purifying spell to prevent Dracula from possessing that area again. Once every area has been purified, his physical body reappears and you have one final battle against him in a weakened state. Of course, the characters fighting will include: Julius, Alucard, and possibly a Belnades spellcaster. Once this weakened Dracula is beaten, the priest puts and enchantment on the vampire killer which Julius then delivers the final blow and then runs off to hide the whip. When Julius returns, everyone flees the castle. At which point, the priest casts one final spell that seals the castle in the eclipse.
The battle is so intense that Julius collapses and is stuck in a coma. Alucard decides to stay awake in order to keep watch while Julius is in a coma.
I have one final suggestion and I know I could get yelled at for this idea. It is to have the final battle be RPG-style (similar the battle system in the Final Fantasy series).