Head left. You will need to find a switch that you should strike to open the passage to Van Helsing. En-route, one of the enemies is a wolf that requires three hits to destroy. If you get injured, you can optionally head further left to retrieve a potion.
Van Helsing can be reached by taking the middle corridor. When you see the floating table, you can destroy the blue specter and jump on the table to reach him. Van Helsing recommends the wooden stake, which is found on the left of the room he's found in.
There is a 1-up at the top of the room, up and left of the exit.
The boss area is unique in that it includes a potion that you can use to heal yourself. However, it also has pits that damage you if you land in them instead of the platform.
The enemy will fly around back and forth. Occasionally, she will stop above one of these platforms, land, then make a projectile attack.
The steak can be used to injure her, but if you need to use a melee weapon, there's a small area where you can land on one of the platforms to make an attack.
First, follow the top route. This eventually leads to a few spear traps, followed by a switch that opens the boss area.