This boss has a simple pattern but does a LOT of damage if you don't avoid the fireballs!
1. Hit him until he opens his mouth. Jump on the first platform to avoid his firebreath.
2. Attack him again until he raises his head. Use the platforms to avoid these fireballs. They do a lot of damage. Even with the best armor equipped!
These two patterns repat and he drops a random fireball from the ceiling during each attack.