If it's the yellow shirt girl I'm thinking of, after beating her 3 times, you go back to that giant woman at the cave at the beginning of the area. Talk to her 2 or 3 times until she moves, then you can fight her near the boulder to the north of the initial save in the area.
Surprised you actually beat her 3 times without knowing about the black bunny girls, since I thought you had to get past a lot of them for the later yellow shirt girl fights, but you have to be completely clean to get past them. If any of your party members have a stench debuff or you have the bunny smell, they won't let you past. Have to go into clear water to get rid of the bunny smell a lot of the time in that zone.
On the beach, if it's the girls out of combat who just instakill you when you try to pass them that you mean, you need a scent from a giant girl that's standing around the northish path near a save in the area, I think. Can't walk into clear water after getting the scent before going past the girls that suck you up as well (not sure if beach water is included or not). Also at least more place later on that gives a scent that gets you past them. Do note that the beach area is optional, though. It's said in the first message when you get there, but since it's a different language, just warning you since you may not know.