To unlock Bowser's Nightmare, beat story mode with any character on any difficulty.
Super Duel Mode Secrets
In order to unlock the locked Battle Games, you must complete the game at the highest available difficulty. In order to unlock a new Battle Game, such as Flag Competition, you have to complete the hardest level of the game before it (in this case, complete Battle Competition at hard to open Flag Competiton). To unlock Donkey Kong as a driver, beat him in any Super Duel competition on hard difficulty. There are also some locked machine parts. If you beat DK in any hard Super Duel competition, you unlock the DK Body, DK Engine, DK Tires, and DK Weapon. If you beat all three games in all three difficulties, you unlock the Skolar Body, Skolar Engine, Skolar Tires, and Skolar Weapon. After you purchase all 44 machine parts, Bob-omb Wall and Bowser Punch become unlocked.
Unlock Frightmare
To unlock Frightmare, beat Bowser's Nightmare in Story Mode.
Unlock Hard Difficulty
To unlock the Hard difficulty setting for an indiviudal game type, beat the game on Normal.
Unlock Intense Mode
To unlock Intense Mode, beat the Story Mode on Normal.
Unlock Mini Games
Once you play a mini-game in Story Mode or Party Mode, it becomes unlocked. In Mini-Game Mode, to play Mini-Game Circuit, you have to have unlocked one four player game, one 2 vs. 2 game, one 1 vs. 3 game, and one duel game. In order to play Mini-Game Decathlon in Mini-Game Mode, you have to have unlocked Chomp Romp, Dinger Derby, Flower Shower, Ground Pound, Hydrostars, Later Skater, Leaf Leap, Night Light Fright, Triple Jump, and Will Flower.
Taunts
Press L during game play when it is not your turn to taunt.
Defeating Bowser
Use the following trick to defeat Bowser at end of story mode. Bowser starts by sending lots of fire-breathing Koopas after you. To defeat them, jump on their heads. However, make sure that they are not about to breathe fire or you will lose a heart point. This is followed by three fire rings that can expand, contract, or spin in any variety, and you must jump over them. After that, Bowser breaks out of the screen and begins to attack you by jumping on you, running into you, or breathing a lot of fire. You can avoid all those attacks by running around him in circles, but do not get too close or his spikes will hurt you. Notice the blocks that he jumps on will start to break; after he jumps on the same one three times, he falls through. Finally, he drinks a potion to make him huge. The red fireballs he throws out of his right paw can be picked up once they cool off. However, if you wait to pick them up after he starts to shoot fire, they will be reactivated and you can pick them up and throw them at him by pressing B button, by jumping and throwing on the little circle in front of the ledge near his nose. Do this five times, and you will succeed.
DK Parts
To unlock DK and his parts at the shop in super duel mode, defeat DK in battle mode under the hard difficulty setting in super duel mode.
Skoler Parts
To unlock Skoler parts in the shop in super duel mode, win Battle, Flag, and Robo Rabbit Hunt under all difficulty settings.