To beat the Tentecula, run back and forth and make him spit poison at you. Use Diffindo to cut off one arm then continue to run back and forth. Cut off the next arm. Immediately cut off the head and shoot Incendio (power up for a few seconds, but not too long or the arms will grow back) at him. You should defeat him in a few shots. There are two Tenteculas.
Defeating Devil's Snare
The Devil's Snare is exactly like the Tentecula, but much easier. Cut off his arms and his head then shoot Incendio at him. This time do not worry about running back and forth. Stay still and he will not hurt you much.
Defeating the Mutated Gytrash
When you enter the chamber after the chess board, there will be a short intermission sequence about Professor Quirrel. After that, he releases about six Mutated Gytrash. In order to defeat them, you have to use Lumos (power up, shoot it, then move towards one or more of the Gytrash). Make sure that you are almost touching them. After you have defeated one of them, Quirrel will try to bring it back. Since you cannot hurt Quirrel while the protection (blue wall) is around him, wait until he puts down the protection. Immediately shoot Flipendo to stop him. Be on guard until he puts the protection up again. Then, resume defeating the Gytrash. Watch out for the Gytrash's "echolocation". If it touches you, it will knock you down and stop whatever spell you were performing. The more Gytrash you defeat, the harder it will be to keep up with Quirrel and the other Gytrash.
Defeating Trolls
You have multiple encounters with trolls. Find a Spongify tile and shoot Spongify at it. Wait for the troll to run at you. Then, jump on the tile towards the troll. He will hit the tile then hit himself on the head.
Defeating Voldemort
After you defeat the Gytrash, there will be an intermission sequence with Quirrel. Here, you will first see Voldemort. The battle will begin when he uses Wingardium Leviosa. The entire battle is worthless. You can even defeat him without being hit once. There are two parts to this battle. This part is optional, but very helpful. Run behind a pillar that holds the ball the Voldemort uses Wingardium Leviosa on. Voldemort will follow you and begins to cast the red spell. Immediately as he fires the spell (when it comes out of his wand), run away from the pillar. The spell will hit the pillar and destroy it. Repeat this with the other pillars so he cannot use Wingardium Leviosa on you. In the required "real" battle, run in front of the mirror to where your parents are protecting you with the white band. The goal is to reflect the spell at Voldemort (as instructed by the intermission sequence). Let him cast the red spell at you. You will turn green, then the green fades to white. The remote will begin to shake. When you are fully surrounded by white, face Voldemort and press A. The spell will shoot at Voldemort. This hurts him a lot. Repeat this until Voldemort is gone, then enjoy the intermission sequence. You wake up the next day in your dormitory. There will be an intermission sequence with Dumbledore. After that, go to the Great Hall. Take advantage of the last day. Do not enter the hall because this is the final area and you cannot save the game once you enter it. Explore the school and get all the Wizard Cards, etc. Then, enter the hall for a final intermission sequence, and the award for the House Cup.
Extra Chocolate Frogs with Voldemort
If you have used the three frogs in the battle before Voldemort (Gytrash), do not worry. You can get these again. Save the game in a safe area in the middle of the battle. Then, exit the game and reload it. The frogs will be there again.
Secret passage to the 4th floor
Go out of the common room and into the main room. Turn right and crawl under the bookshelf. This area is also in the Chamber of Secrets.
Invisibility Cloak
This cloak is not too helpful. Prefects will still catch you. The most you can do is not touch the Prefects. Filch can usually find you, even if you do not touch him. If you do get caught, even without the cloak, you will not get House Points taken away from you.
Gargoyle statues
There are mean looking statues of gargoyles all over the teachers' quests. Light Lumos in front of them and they will reveal a hidden path.
Hidden items
Search all of the desks in the classrooms. They contain Beans, Cauldron Cakes, Pumpkin Pasties, Stink Pellets and even Wizard Cards.
Quest warning
If you are doing a small quest, like following Hermione or Ron, follow them. Do not take any shortcuts. At night there are not any Prefects outside the common room.
Hidden Item
If you go into a room such as the one in the dungeons at the side of Snape's classroom, hit all of the glass jars (the ones that give you bertie botts every flavour beans) and a golden chest will appear somewhere in the room containing a/pack of wizard card(s).