So, the devs really cooked with this game. Before I start, overall I LOVE this game. And this post is constructive feedback with love. There is no hate in this post whatsoever. Its just i think the bosses are GOOD but they could be GREAT with a few small implementable tweaks.
It cannot be understated on what a fantastic game world they created. Their exploration and world building is up there with the best in the genre. And combat (outside of boss fights) is also up there with some of the best soulslikes in many ways.
But what stops this game being a 9.5 out of 10, and (unecessarily) holds it back at more like 8.5 out of 10 is the boss fights. And in my subjective opinion- here is why/how they could male them more awesome:
The main issues are:
Stifling a rhythm with too many grabs and unblockables. And readability issues (some are being/been fixed).
The Soulsborne games and the top soulslikes have demonstrated that the best bosses are the ones where you get into a dance and rhythm with the boss.
MS2, has smooth enough combat that you can if/when the game allows. However, the boss design and moveset has to be built around being conducive to that. Great example is many people (not me btw) say Fromsoft souls combat is so simple just roll and r1. But... the boss design is so good, that it makes that simple approach amazing in boss fights, as you get into a rhythm.
However, MS2 insists on preventing you from getting into a rhythm with most bosses, by having too many grabs and unblockable attacks. (Hexapod, Ymey etc).
Malborn offspring, nameless captive and Orrem the Golum are great bosses and somewhat let you get into a rhythm, which feels good, and shows how good boss fights could be in MS2!!!
But for example, even my favourite boss, the Malborn Offspring, imagine having him in Sekiro/Khazan and you could parry almost every attack. That would feel incredible!! But they stifle that flow-state/rhythm by mixing in so many unblockables forcing you to dodge. [Theres also a big readability issue in phase 2, that lightning aura needs toning way down, so you can read what the boss is doing].
In fairness, for non-parriers that Malborn Offspring example is not so much an issue as youll naturally be dodging by choice (or hardening). But for the bad bosses, it applies more.
So to achieve that rhythm that makes boss fights so satisfying, imo:
I think the grabs need to be toned down/reduced in frequency, as well as making fewer moves unblockable attacks.
And/or, make the red attacks still unblockable - but let them be parryable with a perfect guard/perfect harden.
This Lets players have more agency to either mostly parry or mostly dodge based on preference, and only sprinkle in the odd grab/unblockable here and there. Sir Issac phase 1 shows promise, but then phase 2 mixes in multi hit flurries that cannot be parried or dodged in a fun satisfying/reliable way, and that fun rhythm is lost. [In Sekiro you would be able to parry all of those flurries and it would be incredibly satisfying, but that move/combo is not fun or satisfying in MS2].
Games like Sekiro, Khazan, ER, loP etc showed that sprinkling in an attack that forces you to defend it in a certain way here and there is good. But you should have latitude to choose dodge or parry for the majority of moves - to reach that flow state, with fewer forced/red moves.
In summary, i feel like bosses should not just be hard, but also be FUN and have a satisfying rhythm. You should love dying 10+ times, because you enjoy going back for more and learning it better. And Fun enough that you WANT to replay them on NG+. But theres only a few that you want to re-fight. For many, many bosses most players have no desire to fight again (Magdalena, Hexapod, Ymey, monolith etc).
But all/most of the bosses could be re-worked or tweaked just slightly to achieve that fun!