Dagron, aka the Massively Misspelled Monstrosity, is the fifth boss of Shantae and the Pirate's Curse. He is fought in the Oubliette of Suffering.
Avoid the fire breath either by running to the closest edge of the room. If Dagron is facing the opposite direction, hold Shantae's position.
Be wary of Dagron's tail.
Use the sword-dash to hit the switch on either edge of the room to make rocks fall, then use them as stepping stones and hit Dagron in the head when he comes around for another pass -- this is more difficult than it sounds, because Dagron will ram into you if your timing is off. After struck, he will fall to the ground, giving Shantae an opportunity to pound on him for a bit before he recovers. Dagron's tail will eventually destroy the rocks, so ram the switch again once they are all gone.
The rocks will crumble as Shantae stands on them so keep jumping to keep them around long enough.
As Dagron takes more damage, he will get faster and harder to dodge. Maintain the assault until you deal 400 HP damage, then he is defeated.