Although the Daemon Lord Reborn is normally defeated by depleting its health and triggering a finishing sequence while it's stunned, an exploit allows the boss to be stunned as early as the beginning of the fight.
To do this, wait until the Daemon Lord attacks and use the magnetic railing above to get to the opposite side in which it's facing. Make sure to give yourself enough distance from the boss so that it won't counterattack with its tail. Drop down, charge up the bat projectile, and time its release so that it collides with the Daemon Lord's sides as it turns around.
If done correctly, the Daemon Lord will now be stunned and open for a finisher, of which normally isn't possible until several phases of the fight have passed. Although the following sequence will appear strange due to Alucard and the Daemon Lord clipping though objects that normally wouldn't be there, the scene will nonetheless conclude with the boss being defeated and the player being awarded with the Demonic Wings.