To unlock a special game in Flipside arcade, you need to have unlocked Flopside(For this You need Fleep, who is found in Chapter 4). Go to the bar at B1 Flopside and flip. Go to where the arcade SHOULD be.(It's beside the counter). Then you'll see a gold card and I think there is another one too, but I don't know what it does yet. Go to the arcade, which, if you don't know the location of, is at the place where you found the card, but in Flipside instead. The special game has you playing as Mario with a hammer and there will be shells coming and you need to hit them.