How do I create a boss?

[TehWildcard]

I'm creating a new mod called "Jevil's Mod", and I need help creating a boss that behaves like him. You know, shooting out hearts, diamonds, he can pretty much do anything!
I also want him to drop the "Devilsknife" upon defeat, along with some money, ores, and other vanilla items. Can anyone help?

 

[unrealmegashark]

ask mod devs