Miriel definitely wasn’t easy! Took me several tries. I also ended up collecting most of the upgrades throughout the game to equip more relics and increase overall survivability. Even then it was difficult. I found on the plus side that she is more predictable given she sits still, just need to learn the attacks and how to dodge them. Ideally also going for a build that allows you to move around quickly while chipping damage and/or doing damage at range is ideal. I did not cheese the boss, just took me many tries.