New Super Luigi U: Layer-Cake Desert-3 -- Piranhas in the Dark

Take the pipe underground into the dark cave. There are giant Piranha Plants, some of which breathe fire. The fire and the lights on the moving platforms provide some light. The first Star Coin is over a giant Piranha Plant. The second Star Coin is on the right side of a platform, behind a giant Piranha Plant. Kill the Piranha Plant and stand on the platform. It will eventually dip down so you can get it while standing on the platform. Over solid ground is an item block with a Yellow Baby Yoshi. It will help illuminate the area. Ahead are some Fire Bros and more giant Piranha Plants, some of which are on the ceiling or overhead pipes. One of the giant Piranha Plants is hanging from a green pipe. Kill this Piranha Plant (you won't have time to kill all of them, but you can kill some of the other ones) and go through the pipe. There's an 8-bit Luigi behind the Piranha Plant. The pipe will take you to the third Star Coin and the flag.