I felt the same way. I understand that as a boss, I’m not supposed to get upgrades like better armor, weapons, or even new spells beyond a second phase, so it makes sense for me to eventually lose. However, the AI not only learns too fast, but there are so many instances where my attacks clearly hit it, and yet it takes no damage.
It would feel much more organic if I could chain my attacks into combos instead of having to wait for one to finish before starting another. Sure, it might make me overpowered, but it would make the experience more fun and give the AI more time to adapt to my moveset. Alternatively, just not having the hitboxes randomly disappear would be a much better fix.