From your starting point, run along the alleys on your left and right sides beside the walls. In the right alley there should be a blue orb fragment. Also, on the far right corner of the trap door should be a pillar for a Secret Room. Slash the floating orb on each side of the area with Quick Heart equipped on Devil Trigger mode that will reveal the Blue Device. After activating the Blue Device you'll be sent away with Demonochorus and Savage Golem to defeat and then your new boss.
Noctpteran is an above ground boss so your swords are pretty much unusable so as usual stick with your guns. It will lay eggs that will spawn larvae unless promptly destroyed. If you get eaten by a larva simply swirl your analog stick to free yourself. Once Noctpteran is defeated, beware for the remaining larvae will continue to attack.
When the battle is over, you will return back to the spot of the Blue Device. From there, just hop down into the trap door and end the mission.