Defeating the Hondrox
When you are fighting the Hondrox without BloodRayne cheats, you must break down the pillars (long wooden objects) and the floor will fall. Do this twice to complete the level.
Defeating Beliar and Wulf
To defeat Beliar, you should have guns, and grenades. The blades do not work on him. Use the Magic Eye to find Beliar's weak point and shoot it. When you run out of guns, shoot the grenades at him. He must be defeated quickly, because if he powers up enough, you will lose. After Beliar is dead, go after Wulf. He will run around the room and attack you. Hit him as much as possible. After a few hits, Rayne will stop and say "That was for Mynce". Wulf will tell her that she is pathetic, and the fight will continue. Repeatedly hit Wulf to win.
Indiana Jones reference
In one of the small corner rooms in the Nazi part of the game, you can break a box to find the Ark of the Covenant from "Raiders of The Lost Ark".