Try to keep all your teammates alive as much as possible. After Flavens implanted its spores in a teammate, use Drill Guard as Jacob if you thought he cannot defeat Flavens in time. Once you defeated Flavens and if you still have your turn available, attack Prima before she resurrects Flavens.
Man, I just can't do it. I tried to do the Drill Guard thing but it never seems to be there as an option after the spores are implanted. I use David's Fire Shot as often as I can but that doesn't come up too often.The most I can ever do is land one hit on Prima but I can't keep anyone living long enough to keep putting Flavens down to hit Prima again. Is there a strategy guide or something somewhere? Maybe there's just some basic techniques I'm not understanding. I didn't have too many problems up until now but I just can't get through this fight.
I downloaded 1.76 yesterday, installed it today...I was very pleasantly surprised that I was still able to load my saved games. It still took a few tries, but Jason, Louis and David finally defeated the three sisters! Took a few tries, but Jason and Blu defeated them, too! Now Ryu keeps getting slaughtered by them. I'll keep trying. Thanks for responding. I do feel at least a little better now that I've seen comments from other people having the same problem, at least it isn't just me.