Thanks for the insightful critique. I feel like I shouldn't take this lightly, especially as I see how much the boss killed your enthusiasm.
So I certainly need to rebalance the boss.
I think most of the work I have to do is managing expectations - stage three is actually quite easy mechanically after you figure out what you gotta do (hint at the end of the message). But as the first two phases are purely about mechanical execution, you'd reasonably expect the third one would be, too.... [EDIT: I actually went and made the stages mechanically easier as well with a few level design changes. Should be MUCH less frustrating now]
So I guess this was a fair warning to me - I should always playtest a new feature with new players, instead of just those who already mastered the mechanics. :D cheers!