On the Pause screen, Hold L+R and enter: X, B, X, B, X, B, B, B, X, X, B, X, X, X. You will hear a sound confirmation if you entered the code correctly.
Bosses 1 and 2
Boss 1: The first boss you should encounter is a fairly large Creeper.Since he can't be hurt you have to avoid him on the slippery floor and grab the 5 keys that are floating around [Don't worry. You can just jump to get them]. After geting all of them, head over to the end of the room and get out of there. If you come back there the door will still be open and a 2 hit Creeper will be there too.
Boss 2: Once you get passed the 1st boss & more than a bunch levels you will finally get to the top of Mystic Manor. After seeing the cut scene with the Mastermind [the prime suspect of the game], he will disappear and then the Black Night will be on the balcony that Velma's on, but she's in a cage. When this battle begins, the Black Night will start throwing axes at you. Do your best to avoid them. If you get down to 3, 2, or 1 lives there are 3 sandwichs lying around. Use them wisely. And speaking of! witchs there's one flying around. Since you don't have the thunder smash [unless you put the All Wepons gameshark code on] avoid her. Well anyway, when the Black Night walks past one of the 5 red swiches press the sqare button to take out your helmet and then charge at the switch. With luck the Black Night will get shocked and now that switch will be busted. Get the Black Night shocked with all the switches and now you have defeated him. Now jump up to the broken fence and get up to the balcony. Go to the end and then a cut scene will happen showing Scooby going on top of Velma's cage and letting her out. After watching Scooby finding Velma's glasses you'll then get the goulashes that let you get in the Cannery.
Dark And Stormy Night Part 2: Help From Shaggy
If you die after getting all the keys and having Shaggy up in the coffins, when you restart the level you can talk to an invisible Shaggy and still have him help you jump and walk around in his arms. You can take him all the way through the level to the doorway.
Defeating Master Mind
He will send out robots and space kooks. Defeat them and when the electric field is down, ram the red switches, Keep repeating this until he falls off his platform. Then, dodge his robots and supersmash them. When the electric field is down again, ram the two red switches and repeat two more times.
Defeating Red Beard's Ghost
Red Beard will send out ghosts. Dodge them, then a ghost of Captain Moody will appear. He will throw his sword. Go to one of the ropes, stay until the knife cuts it, and keep repeating this attack.
Defeating The Funland Robot
Ram him, then super slam him.
Defeating The Green Ghost
Daphne will open coffins. When the Green Ghost hovers over an open coffin, superslam the switch.
Get The Football Helmet
To get the Football helmet go to the top of Haunted Hill.
Holiday Extras
Set the system date on your gamecube to one of the following days for holiday treats:
Jan 1st - Fireworks
July 4th - More fireworks
Oct 31st - Bats on door
Dec 25th - Snow pleasure
How To Beat The Black Night
There is 4 switches when the Black Night is above one head smash it with square. It will shock him on every switch and he's doggone. You must have the Football Helmet to do this.
Unlock Aternate Credits Sequence
On the Pause screen, Hold L+R and enter: B, X, X, B, X, B. You will hear a sound confirmation if you entered the code correctly.