Hope it isn't too late to cast some ideas for monsters and/or deaths.
The idea of a large scorpion enemy is decent, two immediate moves come to mind; a claw squeeze attack that can cut the heroine in half, and the always welcome stinger impalement, maybe by holding the legs first. Depending on how you design the mouth/face of the monster it could eat her as well... I think a scorpion monster in Devil May Cry did that. But I'm not sure.
Another monster that would be cool is a Yamata no Orochi, a Hydra, or simply a snake with several heads; each head could bite a limb and pull the heroine apart, could be all at once, or could be more brutal, pulling limbs off one by one and swallowing them. You could also get off a similar effect with a Cerberus I suppose.
A frog enemy with the usual swallow attack is always welcome in my book, the usual tongue lash that drags the heroine in. Bonus points if it can pull her in both head first or feet first... Of course a frog doesn't have alot of attacks that I can think of; maybe a if it had claws and could walk upright?
Worm/snake enemies are always cool, and one idea is a snake with a mouth that opens four ways, and a horn or blade on top of it's head which it could use to slash and/or impale. If it impales the heroine it could bite at her legs and tear them off as she dangles on the blade. Could also have a simple bite attack that gets her about 1/3rd the way in and chews her up as it swallows her, maybe with some blood either squirting from it's mouth or dripping from it.
Another interesting enemy would be one that impregnates the heroine somehow (I'm thinking of some flying bugs in Blank Blood) the eggs hatch almost immediately and either erupt from her insides, or simply hatch and eat their way out.
I know you said you've already decided on heroines, but if I had my pick of a heroine in a game like this I'd love to play as a busty girl in a playboy bunnysuit. Just sayin'.
Well enough of my rambling... hope some of this is at least interesting.
