From what I understand, the original boss fight was going to be fighting the six beasts which each will hold one of the stolen masks on them, progressively losing their infection/corruption upon hits. Then later on the Mechuta I'll still call him was added, and I presumed he'd be an in-between you'd fight every two beasts you defeat, and the swirling vortex above would roar to signify an attack randomly when Mechuta is fighting, but I'm assuming with talk of phases that he was moved to be a phase all his own after the beasts. And now we have the new shadow beast you're working on, which I now know is phase three.
So if I'm gathering correctly, it's beasts with a swirling vortex above it, and Mechuta, but you plan to swap their phases? I feel like Mechuta is really big, and imposing, so having him be the first phase feels wrong to me if that's the case I feel it transitions well having the beasts made of blackened parts that have the golden masks, then when those are stripped away, Makuta only has his infected Hau which would reflect his anger that he's being bested, and he would stomp around, swatting, and trying to grab you, all while the vortex above randomly roars, and slams tendrils down towards you, his stomping, and attacks possibly causing even more damage to the stage in his fits of anger, leading to a very dangerous fight for phase three when you have amlost not flooring left to take advantage of.