Tenebris Somnia bosses are less about damage and more about reading one trick: a mirror, a trap, a warning on the floor. Here is that trick for each fight, in the order you meet them, with a link to the full strategy in the walkthrough.
How do you beat Vitria?
Bait the monster's vertical attack and step back, then use the mirror to find its hidden tentacle. When it's stunned, crack the mirror; the Revolver is better than the Pipe here because the stun window is short. Step through the portal once the mirror breaks. Vitria mirror boss strategy.
How do you beat Sanguis?
You don't outgun him. Let him smash the ordinary mannequin barriers, then time his lunge so the corridor's retracting blade impales him. Never let him catch you charging down a narrow corridor, because that hit can kill you outright. Sanguis chase strategy.
How do you beat Hermes?
Runes on the floor show where each giant hand will rise, and the hand's eye flashes before it fires, so shoot the hands and dodge the blast. When Hermes's face comes out of the movie screen, run as far away as you can until the inhale ends, then hit him. Finish him up close with the Aurifer once he's weak. Hermes final boss strategy.
When do you fight Lacuna instead of Hermes?
If you spare every Relicta Ghost before entering Septimo Arte, the final boss is Lacuna and you get the good ending. Kill even one ghost and it's Hermes. You fight Lacuna without the Aurifer, so bring plenty of shotgun and revolver ammo. Which final boss you get.