-After beating her up until her life is gone, use the Hellfire Gun before you die.
-Make sure you have Fire. If you do not, you must start from a checkpoint with Fire (three checkpoints back). Make sure over the course of playing the game that you have not put any Fire into your crossbow bolts before starting over, The crossbow will instantly kill her.
There is an area just after you enter where you are in the rafters, above a slew of vampires. Instead of taking them out with the crossbow, you can take them all on at once. Before doing this, make sure you get the Hellfire vial from the furnace on your right. Go across the rafters until you are directly above the gate that closes on you when approach it. Walk out on the rafter until you have a clear view of the gate, the crates (also below you), and the space between them. If you aim correctly, you can avoid the crates and fall directly in front of the gate. The damage taken from landing on the crates is easily fixed by an Elixir, or can be used as an extra challenge for those using the training. Land in front of the gate, and the intermission sequence closing the gate will start. You will take no damage from the fall. There are six vampires down here (seven if you count the one that appears when you open the door). Take them all on at once for training.