I think most of the fights were alright.
The boss-fight for the rabbit quest was defeated easily enough (I think around 5 turns, without acquiring enough points for the ultimate)
Whilst this was done on NG+ with 400% XP it wasn't much more difficult on my first run.
I also didn't have to use the water-ability. (or better: the bosfight would likely have lasted longer if I did)
The Fairy-Boss was rather difficult, through likely because I mostly ran into larger groups which made leveling up hard.
This should get easier with the change in encounter size you mentioned through, so I can't really say how bad will be with those changes implemented.
As for the Manticore Boss, it should definitely be doable with decreased group sizes, as long as you're able to get past the robots of course.
The Slime boss was a little bit difficult. Nothing a few more levels shouldn't have helped with but I defeated the ones reachable before getting the tf. I'll test defeating it with a bit more xp on my NG+ and see how that works out. Depending on the result I'd recommend adding a few more enemies (for this region I don't think reducing group sizes is necessary since the encounters weren't that hard)
I can't say anything about the demon boss (after which Serina joined the party) so it was probably not that much of a challenge.
However, the most problems I had were with the swarm of enemies once you try leaving the forest.
(I know very well you're not supposed to beat them)
It took me level 60 and quite some luck to get through them alive.
The wave defence (Kitsune TF) however was quite easy if I remember correctly, through that was due to having teammates with me.
I think the most important takeaway from my experience with the bosses is that they are disproportionately easier if you bring teammates.
In summary I can't say that much about the Fairy and Manticore fights since those will likely get a lot more manageable with a smaller chance to get attacked by larger groups.
The slime fight should be alright, through I'd prefer some more enemies to get XP before the fight
And lastly the swarm at the end of the tutorial hits like a truck. It's quite hard to survive a few rounds (probably as intended)
Finally, as I read through the skills today I rediscovered the "Fourth Tail". I probably didn't use it (not even once) since 90% of MaxHP is quite a steep price to dodge an attack that is usually cast at the half point mark of the battle (Through I didn't really have any problems with boss-ultimates anyway, especially for bosses one can bring a teammate).
If other questions arise or you want me to test something I'd be happy to offer my assistance once again.
Also let me know if I forgot some bossfights (not counting the demons for form-improvement, they were rather trivial if one brought teammates)