Bowser's Gnarly Party is the sixth board. It has a difficulty rating of three out of three. To unlock it for Story Mode for a particular character, you need to beat the other five boards and their corresponding Story Mini-Games with that character. To unlock it for Party Mode, you have to beat it in Story Mode.
Bowser will claim a space every time a new Star Space is chosen. If you reach him while you're Mega, he'll challenge you to an Etc. Mini-Game. If you lose or you're normal sized when you reach Bowser, he'll take half of your coins. If you're Mini when you reach Bowser, he'll send you to a start space in the middle of the board.
Points of Interest
Bridges
After three people cross a bridge, it will change direction.
Bowser Torches
When you land on a Happening Space, a Bowser Torch will burn one of your items to a crisp.
Mini Mini-Games
Brake Bullet Bill requires you to press 
to stop the bullet bill. You get however many coins correspond to the number he stopped at.
Story Mode
After you beat this board, Bowser will challenge you to The Final Battle! Once you've beaten that, Bowser will say he's had enough and drop your present as he leaves. The credits will follow your constellation's appearance. They provide more back story.