All right, I finished the Calamity mod to the extent that I'm going to, and felt compelled enough to create an account to make this post. Overall, I greatly enjoyed the mod and the vast majority of the bosses felt challenging but balanced. As a veteran of all 3 Dark Souls games and Undertale, I appreciated many of the references in both the npcs and the boss fights. Really, the reason why I am making this post is the current state of Supreme Calamitas, which I feel is problematic. Of course take this with a grain of salt, as always, but hopefully some of this feedback is relevant or helpful...
I managed to beat the old Supreme Calamitas without much of a problem, the most recent version though -- things are a bit absurd in my opinion. I am not really sure the 'bullet hell' phases add much besides unnecessary frustration. I don't think that Terraria is really designed for bullet hell with its janky movement and less than perfect performance. My personal feeling is that if I wanted to play bullet hell, I would play Undertale. That said, the updated boss fight is clearly not impossible, and intended to be much more difficult. From the perspective of a game developer, I understand why the final boss is extremely challenging; it's the last fight in the
modded game, after that there's no more bonus content. However, I feel that in its current state the Supreme Calamitas boss fight a bit too difficult and frustrating with the added bullet hell phases. Just for some context, it took me over 50 attempts to beat the Fume Knight in Dark Souls 2; that was the boss fight that nearly broke me. Yet, I kept trying and the Fume Knight is now my favorite boss in the Souls series. However, after about 10 real attempts at Supreme Calamitas, I feel more like giving up rather than try to master the bullet hell phases. I enjoyed the rest of the Calamity mod a lot and still think it's the best Terraria mod out there, but the updated last fight leaves a bad taste.
If you were to rebalance the Supreme Calamitas fight, you might consider reducing the damage of the hellblasts (and increasing the maximum player hits by 5-10 to compensate), slightly lowering their speed, or simply reduce the number of hellblasts on the screen. I think the biggest problem is that there are times when the hellblasts clump together and are almost impossible to dodge. This leads to massive damage that feels unavoidable, and leads to a domino effect for the player where they can't heal anywhere near the damage they sustain.
Hopefully this has been helpful anyway. I look forward to future versions of Calamity. Cheers and keep up the great work!
(Edit: With high damage reduction, it's possible to reduce the hellblasts to ~300 damage each. Trying to tank them, without cheating, will result in the mod thinking you are cheating and instantly killing you about halfway through the fight. I wish there was a better anti-cheat mechanic, but tweaking the maximum number of hits before the auto-slay is another possible balance option.)