Which means ER is stuck with a heroic sheriff and a monstrous witch. I would try to keep it in the 1500 to 5000 word range but I won't be a stickler on that.
Try to cover the basics of this type plot, define the threat, call the hero (usually reluctantly) into action, as only they can save the day. a few initial confrontations and setbacks make it seem all is lost, then in a climactic confrontation the good guy reverses her misfortune escapes a fate worse than death and saves the world as we know it. You might need a little more than 500 words but I'm sure you could pull it off under 2000.
Have fun with it.
Now Euryleia has a greater challenge and if I were her I'd consider useing humor to light the path. Somewhere between the Python's Holy Grail witches and Niel Gaiman's The Graveyard Book- Chapter 4- The Witch's headstone http://www.mousecircus.com/videotour.aspx- play off all our preconceived notions about witch trials and such. I'm sure she can do it. Doesn't mean you're not a bastard for making her do it, but you did respond first.
Best of Luck
Mad Lews