Overview
How to defeat the first boss with any character.
Use The Roll!
Pick one corner and stick to it. Every time a ninja star is thrown, roll. If your run out of stamina (the pink bar) then jump or move to the side. After every bout of stars there is an opening to attack. Repeat until he is defeated. It can be confusing because the Ninja teleports away but you are hitting him, just stick with it.
Other Bosses
I will be adding strategies for the other 2 bosses soon.