Your main objective is to look for Blue Devices to open the way into the demon world. First, run under the broken arch by the Divinity Statue and turn left to the door after the pillar with the red orb for a Secret Room. Afterwards, continue on to see the first one very close by. Move along and hop over the small door and keep going until you see the second one on the corner. At this point you should be close to the arena so jump in and get the full Blue Orb. Next, get out and run down the narrow path and turn the corner for an another Device. If you head down even more, you’ll face some Demonochorus and Savage Golems but more importantly you’ll get some new swords against the wall, the Zambak. Now turn back and follow the glow of the Divinity Statue until you spot the thin path to the left where the final one will be waiting for you around the right corner on a ledge (opposite to the floating ledge with the Red Orb). In the room beyond strike the eye and let the battle begin.
Phantom uses the same moves as always, a good strategy would be using your new Zambak with Devil Trigger and wreck havoc on his face, though be ready to dodge when he finally attacks. Lather, rinse, repeat until the fight is over along with the mission.