The only boss I found to be unfair so far is Misora. She does way too much damage for a team comp check fight. I had enough damage here, but it took me a while to figure out how to keep Hatsushio to survive for a bit so I do enough damage. I also probably only survived because I rolled Inori along the way, so I guess this boss was still successful in terms of making me use units I wouldn't otherwise.
Nakayoshi X is a fairer example of a team comp check, but this fight gets too easily cheesed by the debuff bonuses.