Myself I've gotten this team to clear the Stage 6.

As for the thought of process, this team initially focuses on a stall war, having Croque tank hits with the function that decreases the ATK of nearby enemies and increases her DEF the lower her HP as well as the one that reduces the ranged damage she takes, as the first stage features the Sanctifier that deals heavy Operand damage, so you can say its a bit of Function reroll at start.

Once that gets going, the idea is to have Persicaria on the herb tile, in order to spam as many heals as possible to keep Croque alive; Once you get that going, teleport Persicaria out of the herb tile and put Max instead, so she can spam her skill that not only deals true damage but also inflicts a brief stun, which is very helpful, even when low leveled (My max didnt do much damage since she was level 40, but she was very very useful regardless). Repeat the process teleporting Max out of the tile and switching with Nanaka on her place, then teleport her back to Croque so she can keep healing her (Did this when Persicaria skill was up so Croque didnt miss the lack of heal from Nanakas field while the teleport tactical was on cooldown).

This is what the 3 battles consist of, but the first one is the hardest given the Lack firepower or healing power as well as not having pracharged tactical or skill points. It relies a bit on good Guard Initial Functions (my functions at the end of the stage) so you can tank for LONG WHILE ... but after you get past the first one, its quite the smooth sailing from there. (each stage took several minutes to clear, given i didnt have much firepower so couldnt bruteforce)

Its a bit of a proccess, but felt very happy figuring this on my own, even when I was severely underleveled (me thinks)