6-9 Nightmare mode problem

I’ve successfully cleared all the stations of stage 7 of Nightmare mode & stations 6-10 & 8-1 with all three stars, several time levelled up all my heroes & equipment, but I still can’t get 3 stars at station 6-9 & more my team might even faint before the end of the battle.

Is it logical?

It’s pretty normal as LUCK contributes a lot in achieving 3-star battles.