This achievement is for beating the boss at the end of
This boss fight can be pretty easy to beat. You must deflect the beach balls back at the boss with your umbrella by holding
, and when he's staggered, that's when you can attack him. When you are trying to reflect the balls back I made sure I was somewhat close as sometimes they would bounce in another direction then back at him.
After about 30% of the boss' HP is gone, he will add a new type of ball - an electric one that looks like a triangle of sorts. Just dodge it instead of deflecting it. Then proceed to deflect the beach balls from before.
After about 60% of the bosses health gone, he will add a ball with eyes along with the other two attacks. Try to lure it off the screen; if the ball hits you or the fountain, it will spawn 2 small bats. In which you will need to kill those enemies as well while still dodging the bosses attack.
After about 90%, the boss will transition to the last phase, and instead of shooting beach balls or any of the other attacks, he will spawn 3 large fire balls on either side that you need to hit and send to the boss. He will also summons Shadows of himself that also shoot orbs at you so make sure you dodge those as well.
After hitting the boss with 3 of the fireballs, you will have beaten him and unlocking the achievement.