Unpopular opinion: I actually like Coliseum 🤷🏽..and here is why.
IMO, it gave a challenging way to get r5 materials that only cost energy and time. No revives or health potions needed.
It being champion specific was unusual, and sometimes annoying, but doable.
My personal issues were 1). it costing 25 energy points per entry. It should have been 15, and 2). the defender often being too passive.
Overall, I enjoyed this new mode that took us somewhere different.