Another note:

It might be worth considering a randomization on which boss comes out when. It would keep players on their toes. Also, the idea of animations between each phase would be nice, but add a lot of wait time, and also add development time to some extent. It's probably easier to just have some sort of dedicated arena that adjusts to each boss.

You could also have an even harder mode where the arena effects stack with each other. For example, if you clear Shiva, then Titan comes out, Shiva's ice wall remains, or permafrost remains and you have to do all of Titan with permafrost active (that would be hell but hey...)