Castlevania: Order of Ecclesia/Somnus Reef

Boss: Rusalka
Rusalka has several different attacks that she telegraphs with her arms. She splashes water on the ground towards you. Either jump or back dash to avoid. She shoots water in the air which you can be avoid by staying on the ground. If she is close she will try to stab you with her tentacle hair. When she drops stone pillars you don't want to destroy them. She uses a tsunami attack that fills the whole screen so you want to stand under a pillar to avoid it. Slashing and lightning attacks are effective against her.

After you defeat Rusalka, continue going right until you get to the end and pick up the HP Max up in the chest. When you get to the surface, go left and rescue another villager, Serge. Break the top entrance of the room to reveal the hidden Vic Viper. Just before the exit, break the statue underwater to get the Vol Ascia glyph.