Yes. You can use the "Enemy Transform..." event command during the battle. Then you could do it by two different ways:
1 - You actually prepare an animation to roll during the transformation (of each one of their phases);
2 - You could draw a picture, just like the Statues of the Gods which will serve only as some kind of parallax so you can use "invisible enemies" to serve as target above the image and as you defeat the bosses, the picture would descend and you use the Enemy Transformation to set a new invisible enemy (to serve as target of this boss) and so on...
I think it does but I used the "parallax enemy" figuratively! What I meant is that you can create a boss like this using a picture (with the Event Command: Show Picture) to ilustrate the boss in the screen and then using an invisible enemy as a target above the picture (to give the picture boss a "form" to be attacked).
I'm thinking about opening a "game in development" thread, though i'm not entirely sure yet. It's still really early in development, and most i have to show are assets or really chaotic map WIPs. There is a lot of story +the world itself though, and i struggle with converting all of my notes into readable text, and a thread might help...?