I was struggling with this at first aswl, but i finally done it. Once you have killed the other two brides of Dracula make sure that you have full health and that you have full amunition before taking on Marishka, if not, then use the health and ammunition increasers from your inventory if you have any. Have your pistols ready with the alternate ammo and begin to shoot at Marishka while holding the targeting button. The alternate ammo in the pistols is great against Vampires and Werewolves which you will meet soon after this level. When she lights up and begins to fly towards you roll underneath her when she is close, she won't hit you. Continue to shoot at her by rapidly pressing the fire button and staying as far away from her as possible but keep her in your sights by holding the targeting button. She tends to use a lightning strike attack also and the only way to avoid this is by getting behind a pillar to obstruct its path or by continually jumping. You will notice that her health bar will decrease rapidly with the better ammunition. So keep rattling off those rounds until she's done and dusted. Good Luck.