Lmao, like you said, maybe you just have a skill issue
I beat all the bosses on hard difficulty and managed to no-hit Globeh, Mimi, Nexa, Zui and B.B.
Impossible is quite literally impossible because it is solely based on RNG yet I managed to beat the bosses on practice mode and even no-hit Globeh (fav Boss I LOVE his theme and attack patterns)
Just keep playing and eventually you'll find a pattern in evading their attacks. You are also likely making the mistake of choosing ineffective/underwhelming cards against certain bosses.
(The guided missiles do NOT work against Boogle Beast and the projectile barrier spell helps in cancelling Globeh's tornado so you can dodge easier and so on)
Also, about the Nexa cup game frustration, you have to pay attention to the music because it is synchronised to the rotation of the plate. It's all on you for not positioning yourself properly just before the plate spins rapidly.
Hope you beat the game anyhow. You should abuse the dodge mechanic as much as you can 'cuz it literally gives you immunity against any attack 👍🏻