Byakko's diffiulty is based purely on how well SE's able to increase the skill of players overtime.

Byakko is easy because much like a majority of content aka dungeons, we are slowly taught mechanics over a large period of time and then those mechanics are integrated into tougher encounters with unique twist added to them.

The game only gets easier because that's the natural progression of things, you know to dodge telegraphs, to understand how certain AoE works, to run away when necessary, to avoid swipe damage, to share AoE dmg etc etc.

Now if you were to imagine Byakko without being aware of even 90% of the mechanics, it would be a lot harder but that's not the case. People go into the trial ready to understand each concept of the mechanic.

On top of that you have Byakko story mode that pretty much shows you all of Byakko's moves so you can be ready to react as long as you know the name of each skill used. It's all a learning process and the end results just go to show SE does a good job at teaching players to be ready for certain mechanics.