You could use instance variables on the boss for what phase it's in. Go to the layout, and click your boss sprite. To the left you'll see where you add behaviors and what not. Add an instance variable called phase. You can use either a number, or a string. Set the starting phase, could be anything, call it "form 1". Then you would just use that instance variable in your event sheet to do what ever you want. You could do something like every tick, add another condition, click your boss sprite, go to compare instance variable = "form 1" -> action: set boss animation form 1. It might just be glowing red, or whatever.
Of course this is a bare bones starting point. You'll want to use variables like this to decide what animation is playing, and use events to control the flow of form changes, and what not, but once you learn what I'm talking about the picture will come a little clearer.
Also as a note you can use events to set this variable to something else. Just make the conditions, the action will be in the sprite, set instance variable..
Variables are your friend for helping control flow, and keeping track of states.