Thank you so much! Reading through your entire review has been a blast, and I'm glad you enjoyed it. I'll answer a few things from your review.
1. Yeah, Marx is way too easy. I wanted to make him accurate, because Marx was one of the first bosses that made me fall in love with boss design, but I made it a bit too accurate, so it ended up way too easy (Especially compared to the hell that was the previous boss hehe). The black hole OHKOs as a bit of a band-aid fix, and the reduced timer was also because he was easy. Marx and Kracko are probably the only bosses in the game I'm not proud of, so I'll probably make a big update fixing both of them.
2. Meta Knight's timer is so short because he used to be easy before the game released, but I made remade the fight from the ground up later (guess I made him too aggressive, sorry), but forgot to change the timer. I'll fix it in the next update, as well as that boss door warp glitch (thanks for pointing that out, I had no idea). Also, the Mach Tornado is completely random when he turns and move up and down, which doesn't really mesh well with the rest of the game, so I'll probably change that too.
3. Boss 7's black hole attack. I'll probably just remove it lmao.
4. I'm currently working on a similar game, and attacks like Kracko's dash attack where bosses move to one part of the arena suddenly, I usually remove their hurtbox until the attack starts, so I'll probably fix that in this game.
5. Some boss attacks being harder than others making some fights very RNG reliant has always been an issue with the bosses I've made. The later bosses in the game follow a different attack pattern, in which they will do a few weaker attacks followed by a harder to dodge attacks (specifically Boss 10 and Boss 11's second form). The easiest fix would be to heavily nerf some attacks, like Boss 5's second phase's skate attack.
6. The poise break system was added later than most other systems in the game, and only applies for Bosses 2, 3, 6, and 11's first form, although I might add it to Bosses 10 and 12 as well. Its more of a hidden feature, although I guess adding a tutorial for it wouldn't hurt, and it pretty much works like this: If you attack one of these bosses relentlessly, with little time in between hits, the boss will be stunned for a few seconds. It's mostly a feature for when you get really good at finding good opportunities to attack, and is really useful in timed/danger/arena battles.
I never found your review too harsh, and all of the complaints are very valid, and I'm glad you enjoyed Boss 2 and 12 as much as you did, since I put a lot of effort into those two bosses in particular (as well as Boss 10). I like how you pointed out the final boss is similar to NKG, as that was obviously the main inspiration lol. It's especially easy to notice with his Jet attack, which is almost identical to a NKG attack. Also, I completely understand if you don't want to do the Arena as it will probably take a lot of practice, but there is a secret 13th boss, as well as a even more brutal version of a certain other boss as a secret-secret final boss. Also sorry for all the bugs, Clickteam is such a fickle engine, and I didn't have any playtesters other than myself, so whenever someone points out a bug, it's very much appreciated so I can fix it in the next update.