Save the game after completing it to unlock all bonus materials.
Defeating Elder God
Whenever a tentacle has little blue dots floating around it, quickly slash it. Watch the eye occasionally when the blue shield is gone. Get in quickly and give it a good slash. This is the only way to damage him. When the eye starts to shoot out different colored balls, go to the right side of the screen and stand in the depression in the ground. Run circles in it, and most of the balls will miss you. Dodge all of the tentacles as they whip around. Use these tactics and you should survive.
Defeating Janos Audron
Like Turel, he is invulnerable except for certain moments. Dodge him when he dive bombs you, then get in close and slash at him. You can also attack him after he has shot the orange tornado at you. Attack whenever he is down on his knees.
Defeating Kain
Use the basically strategy when defeating Raziel as Kain.
Defeating Raziel
As Kain, use your combos and especially Kinetic Shackles. Pull or throw Raziel around and keep slashing. Raziel is quick; keep moving and use all your skills.
Defeating Stone Statues
A bit into the game you will start to encounter stone statues that come to life and attack. An easy way to defeat them is to exit combat mode and run close to it. When it starts to attack, get out of range. It will hit the ground and be on its knees for a short time. Jump near it and press Y to do a mighty chop attack. The back of this creature tends to be its weak point. This will get more difficult depending on how many of them attack you at once. It seems that the most that will attack at once is three.
Defeating Turel
You will take your vengeance upon his monstrous form. He is almost invulnerable unless you do the following. When the glowing yellow bands surround you, find the gong that is emanating them. You have to hit it either with your Reaver or with a Tk bolt. This will stun him. Get to him and hack at him as fast as possible. Repeat this as needed. Watch out for his other attacks. Near the end, all four gongs will start to emanate. The two end ones should be hit, killing him and giving you stronger telekinesis.
Finding The Earth Relic
When Kain is looking for the Earth Relic, hunt in whichever forge he is in. When you find a hallway filled with black Spectres (with purple eyes) run along it until you see a fence. Run in a half circle on the left side of the hallway. Jump over and down to find it.
Respawning Blood/soul Basins
When playing as either Kain or Raziel, you will sometimes find Blood or Soul Basins which will recharge your health when you consume the contents. These respawn somewhere between two and five minutes apart from when you used them. This is very useful if you are locked in a room with multiple enemies, such as statues, which do not give either blood or souls upon their death.
Spirit Forge Puzzle
When you get to the Spirit Forge and the Elder God is closing it of with his tentacles, do the following. First, jump down and slash where the four connect. Next, climb out and use your Dark and Light Reavers in the holes on the ground (beneath the glowing runes). He will shout and drag the front deeper. Go down and slash. Use Earth Reaver to help get back up to the top. Do the same thing for Water and Fire. He will retreat deeper. Slash him again. Use Earth to sink and slash, Water to freeze the waterfall, and Earth to create to rock bridges. Climb to the top and stick the Air and Earth Reavers in to ignite the Forge. Then, just float down and get it.
Traitorous Enemies
If you get in a fight with multiple enemies, a strike that is meant for you may hit one of your opponents. Sometimes two enemies will flat out attack each other and forget you. This is very helpful when you are fight multiple stone statues.