We find out in the first ten minutes that Mary has strength comparable to Jason Voorhees, Michael Myers or any other standard slasher. That's the only way she could turn a woman's head completely around on her shoulders with no visible effort. Fine, that's the kind of movie we're watching, my disbelief is suspended. Alissa seems to have the same strength, what with bisecting a teenage head with lightsaber neatness with a pickaxe. Again, fine. So where does that strength go in the final confrontation? One punch, and she's no stronger than any other rather small (if vicious) teenage girl. Is getting punched in the face her kryptonite or something?