Hint: Remember to keep shifting planes, from material to spectral. When you run into a wall, shifting from the material plane to the spectral and back again is almost always part of the answer.
Hint: Everything is a weapon: bonfires, torches, spears, pipes, wall-mounted spikes, shafts of sunlight, pools of water, and so on. You'll want to explore every possible means of killing your vampiric and human foes, not only to get from puzzle A to puzzle B, but also because it's fun!
Hint: The special abilities you gain from killing the big vampire bosses (there are five) are necessary for completing the game. Every time you defeat one of Raziel's brothers, try to apply the power you've gained to new puzzles--and to the old ones that stumped you before.