I have played a different version some months ago.
I do not know whether it is a bug, but in the first boss room, I might have gotten stuck in strafing mode, such that I only exited strafing mode while holding 'space' or 'shift'.
When I first entered the boss room, I had a huge number of charges, maybe 42. When I died and respawned (I had watched the boss battle on a stream, and I got much farther on my first attempt than the streamer's first attempt, in large part due to charge as well as seeing others try it), I ended up with many fewer charges, namely 10. I assume that is a bug. So I quit the game and tried from the beginning again, since having many charges would make it much easier.
On my third try (I aborted the second try very early on once I discovered that I only had like 10 charges), I managed to beat the shield boss. I had zero health/shields left. And 6 charges, apparently. I think I entered with 45 charges, so I might have spent 39 charges. Ignoring difficulty and when it should be encountered, I like the boss.
The controls and movement are atypical, and players need to get used to them first, so I think the boss should be weaker or a different boss should be there or something. Or longer training levels.
I like the levels a lot, aggression and offense is a necessary tactic in some of them.
I like the second boss, finished the battle with 4 shields. I did not use any special weapons (I think I had the axe weapon).
Now I am at the challenge zone.
I finished the 4 challenges, those were fun. There was no way to exit the challenge level selection from what I could tell, so I exited it by quitting the game and starting it again.
It would be nice if you could exit to the main menu instead of only exiting the whole game.
I barely managed to complete the "unfinished, unfair" level - the last part I just rushed past the enemies.
I liked it a lot overall, I do not know if I would have been able to defeat the first boss using just 10 charges though.