I actually have some experience with this, as I make all my significant others cosplays, and they most recent one was Morrigan from Darkstalkers. We found the best way to get the hourglass shape under a leotard is by wearing a full body shaper to slim down the waist, then hip pads on top of that, under nude control top pantyhose. Then for her, since her body shaper was black and showed through the nude hose, we layered black tights, and then Morrigan's purple bat tights, then the leotard. For Raven, you could easily buy any type of full body shaper, as her leotard has a lot of coverage, and you wouldn't have to worry about the straps showing. We were more limited, having to go strapless (which I would advise against, it makes your forms sag). Look for one with firm control, and boning in the waist area.
"I don't have to conform to the vagaries of time and space...I'm a LOONY for God's sake!"