You are fighting Coral Riff in a water-filled arena. Defeat the tentacles, then kill the Octoboss.
The tentacles will come after you two or four at a time depending on their health in later Zones. Each tentacle surfaces after 4 beats around the player and will attack cardinally. The tentacles will shake on the beat before they will attack. Each will either die or teleport away upon being hit. The tentacles will resurface a second time if you miss attacking them the first time.
At the second phase, any undefeated tentacles along with the boss will now come slowly towards you. The Octoboss will begin to shake, then move forward, attacking all squares around it on the tile it moves to every second beat. The tentacles cannot move while attacking.
In the Zone 1 variant, the Drums have 2hp and the Octoboss has 3hp.
In the Zone 2 variant, the Drums and the Horns have 2hp and the Octoboss has 4hp.
In the Zone 3 variant, all instruments have 2hp and the Octoboss has 5hp.
In the Zone 4 variant, all instruments have 2hp and the Octoboss has 6hp.
DLC In the Zone 4 variant, all instruments have 2hp (like Zone 4) and the Octoboss has 7hp.