Key to beating the boss:
patience. Wait for a gap so you don't get hit by the spike shield, and dodge the star he throws at you. He should pause for a few moments before he throws another, so take that chance to jump at him.
After that, run out of his line of sight. Wait until there's another opening in his shield and continue striking until all the balloons are popped.
When the balloons are popped, get OUT of his pink explosion range. Repeat until he's defeated.
Also recommended: stock up on a few of the healing berries. You'll probably need several. But after that, you should be good to go.
Anyways, good demo! I liked the in-depth customization feature.