To break Dandrobium Guts you need react and counter (not necessary on the same team).
I made this special squad for Dendrobium/Black Baccara Aqua Shadows:
Flower Knight Tactica-asteam-5794dd0.jpg
I send teams 1 & 2 on Dendrobium and 3 & 4 on BB.
On team 1, Tritonia is here for mandatory Negate damages. Queen Japanese Anemone and Flame Lily are here with Taunt FM to take hits, Guts and buff skill rates and damages. Silk Tree gave Crit rate, Crit damage and react for Balsam. Balsam is here for insane damages.
I choose Balsam instead of RRose because I need multi hit skill. It deals more damages and it may activate several guts but I'm not sure about that at all.

Team 2 is here to break the guts wall. Apricot bring Negate damages. Flower festival Lantana is here for an extra x1.2 skill rate for every teams. The last girls take down the guts with their attacks and counter damages. And can survive with their own guts.
Counter is only useful to break Guts since Dendrobium take only 1 counter damage. But it's enough when she stand with 1 HP anyway.

Team 3 is my very classic wall team. They just take hits and bring Black Baccara to her berserk mode. I use Easter Cactus instead of Saffron for heal/revive just to make even more damages. BB defense power is absurd and I think it's better to fight her first 50% HP with a defensive team instead of a Crit glass cannon.
Easter Cactus should not have Taunt FM but I forgot to changed it (I used Saffron before I realize Easter Cactus is better here).

Team 4 is overpowered and is here to finish BB. They are supposed to take some hit and don't die in one turn since BB also have some guts.

To be honest, I killed the two girls only once and i did the 25 clear by killing only Dendrobium.
The recent stages are very hard and designed to be a challange for very good players. That's normal if you can't clear them right now. unless hard stages from before Nushi times, full team of 6* are not enough at all. Now you need precises 6* and precises FM. And even with that it's hard and not thought to be cleared in one try.
Although, Dendrobium & BB are a bit easier since you don't need over complicated strategy and order to beat them (like the previous Aqua Shadows). You just need absurd super power. And you're not that far from it with your recent girls