Something I kind of forgot to mention, everyone should have a desire. Weather it be love or gaining power. Also, the weakness isn't super weak. Think of it this way:
During the chaos that the Dungeon Lord caused everyone was like high level. Pretty much all the girls were blasted back down to level 1. So like the more they fight and travel they'll unlock their powers and evolve into more powerful forms. Maybe even discover new ones.
The first major fight will be with the Minotaur guarding the prison at the end of it. He'll have evolved into his second form so the reason they couldn't escape was because he was too powerful. Also, for plot purposes, is it alright that I have the Minotaur reveal the true identity of the monster girls to Cress while they're fighting him. It would just fuel his desire to defeat the Minotaur even more. :3
I figure the battle between the Minotaur will be one vs one. Kind of like in shonen anime, so we could like give more skeletons or monsters to fight for the women as well. We also could work out some generals/other villains for the girls to fight to have epic 1 vs 1 battles later if you wanted. :3