How do I code a boss that has different sprite segments, plans to have different attacks and phases?

A boss I'm making in my game for a competition is a complicated one in theory. I want it to have different segments, two separate hand sprites and a head, and have alternating attacks before showing a weak point for the player. I also wish for enemies to be summoned when the boss is hit and enters a new phase, but I do not know how to code such a string of events and have not found anything to help assist me.