Hm, all of those ideas for changes would be mostly after the section you're stuck on. Maybe the issue is you missed some info in the tutorial popups because of the small text.
The intended solution for that fight is.
- you need to stand next to the boss (put your threat on them) every turn to prevent them from healing.
- you need to Brace! some turns, try to keep some block for next turn. Then not Brace! next turn. Because it costs your reaction, so you won't deal any damage.
Do you feel like you're doing that and it's just too hard? If not, which of those things is new to you?