I had fun figuring out the timings on deflects. The best part of that was realizing I could get a better reward for counterhitting the boss. It reminded me of God Hand's counter hit system a little although much more subdued in terms of what I get as a reward since they still continued their combo sometimes. To be honest these two tools are so powerful I barely engaged with the rest of the mechanics you had. The only point I found for using the projectile was to teleport to the boss at the start or do deal damage to the knocked down boss after a riposte which I felt couldn't be intentional (but should totally be kept in). I've never played Sekiro though so I don't know if it's the same in that game. I'd definitely be careful with making parrying the only strong mechanic since it feels so passive.
I only found out that you already mentioned the AI was random after I had beaten them so I was prepared to talk about how inconsistent their behavior was. I couldn't tell what the difference was between her forms since they all tended to use all 3 weapons the same way. I also couldn't figure out their poise which made me play way too safe and barely ever attack since I figured she would just armor through everything unless she was specifically in Dark Halo mode charging into Human mode.
The poise being so obscure made me never want to commit to longer attacks or try out other mechanics out of fear of screwing myself over. Maybe her poise could only apply to the beginnings of specific attacks (like the great sword) so I could be encouraged to whiff punish still.
I'd like more responsiveness out of my other evasive moves like jumping away and running, I felt so suckered to the boss in the current state of these moves and opted to just stand there and deflect instead of doing anything else. Might just be my bias towards games like Ys or Wonderful 101 where I'm running around the entire arena while fighting, probably the benefit of having a fixed camera angle so far away I guess.
Overall I like the idea of the game and I'm interested in what comes next, especially if more comes out of interrupting the bosses.