That's something you can clear by having a difference of around 2k bp max and with a healer but the next problem is 9-2 which always shows battle error
Are you using above 10 multiplier? I had that happen in 9-2 when I used 11 multiplier, this issue doesn't happen if you are under 5 multiplier with max 1k difference in bp. I have been using this on story mode only so far under 5 multiplier so didn't got any problems so far besides 9-2 which I had to level up with bp