When you come to Fluffy, play the flute (press the L button) to him. NEVER stop playing it, down at the bottom of the screen, there is a bar thing, when it goes blue little by little, that means that Fluffy is getting tired. He will zonk out after a while and then, just move to the back and there will be the trap door to go through.
For the Devilsnare, just go around all over the place. Everyonce in a while, you will come across one of the plants shooting things at you, next to it will be a bowl of fire. Cast Flipendo on the bowl and the plant will die.
For Snape's part, go to the room in the southeast first, then complete that and go to the one in the northeast, then the one in the southwest, and then the one remaining (northwest) when you get all of the potions, go to the cauldron and put them in (press A)
The flying keys room is like a quidditch match, just follow it and then catch it.
For the chess part, whenever a chess piece goes over a button, cast Flipendo on it and it will freeze. Do that until the are all frozen, and then ron will take the last button.