The only difference between Bael and Dagon is that Bael's Rusalkas are blue, and Dagon's Rusalkas are red. Seriously. They have the exact same attacks, and the exact same strategy works. Therefore, I shall call him DAEL and save my breath.
Official Game Guide explains that after NERO has...
I have tried this several times, but alas Dante has yet to emerge in Nero's place. ~_~
Devil Triggering out of a Rusalka's embrace causes Dael to withdraw them. His red eyes light up in the darkness and he lunges at you with full force! Jump out of the way or he will swallow you and start chewing for massive damage. If you are swallowed, or about to be swallowed Dael, Devil Trigger immediately to break free. Strike a balance between using Devil Trigger to inflict damage on Dael, and conserving enough of it (at least 3 full bars) to bail you out of emergencies like the Freeze Hug and Swallow.
While the Rusalkas are out, Dael may drop large chunks of Hail from the sky, so be ready. You can get Dael to come out of hiding by striking a single Rusalka until she loses her glow, or by escaping out of a Freeze Hug with Devil Trigger. Dael will charge you, so leap out of the way and counter him with attacks to the head while he recovers. The same works for Squash: get out of the way and counter. As Dael hops around and attempts Ice Missiles or Ice Quake, keep shooting at him from a distance.
When you've done enough damage, Dael will collapse and roll out his tongue. If you're NERO, run towards him and tap Buster to initiate a special move for massive damage. If you're DANTE, Devil Trigger and unleash every combo you can imagine.
Sometimes Dael will
freeze himself and become invulnerable for a little while, but you can
still slash or shoot the inert Rusalkas to yield red and green orbs. In
the last third of the battle, Dael will glow red and his attack frequency
increases, but he follows the same pattern and the same strategies work.