I just kept on trying out different combinations, but there are a few key takeaways. Don't let anybody get downed and once you get the flavens, take out Prima. You can attack only when you still have your turn otherwise Prima will resurrect the flavens and this is why you have to make sure that you still have everyone available.
Also use drill guard as much as possible (except once you get through the flavens, switch to an all out attack instead, but keep the above in mind) and heal early (it also gives you more energy to work with).
Also it seems to me as though the outcome of the battles is heavily dependent on luck (RNG - damage, party member being attacked and such). Especially with the flaven spores being implanted into a random party member.
Also if you're more of a math person, keep in mind that every turn you get 15 energy and potion gives you 30 energy.