Most of the Colosseum battles are badly tuned imo. Either they're way too easy and only require a repetitive pattern of swing -> move -> block - > swing etc. or they're very badly overtuned like with the Chimera where you literally have to get a few lucky dodges or you can't win, or the Nursery Crab where the placement of the eggs, their seemingly random spawn timers, and the touch damage with few invincibility frames makes that fight ALSO entirely luck dependent which makes both of them very frustrating. There are a few others that need some TLC but mostly at the moment the Colosseum gives very little reward for the time investment and grinding boss fights at lower tiers gets boring rather quickly.