New Super Mario Bros. U: Superstar Road-8 -- Pendulum Castle

This is a treacherous castle. Spike balls on spike chains are everywhere and there are Bony Beetles to deal with as well. Many platforms are donut lifts, so you can't stand still in many places. The Ice Flower or the P-Acorn are incredibly helpful here. Take your time and learn the timing of everything, and get rid of as many Bony Beetles as you need to. You can use the Ice Flower to get rid of them, or, if they're on donut lifts, stomp them, then stand on the donut lift it's resting on to make it drop. The first Star Coin is under a set of donut lifts in a series of ball and chains. Stand on one donut lift to make it drop, then jump to safety once you get the Star Coin. Or, you can use a P-Acorn and avoid the ball and chains all together. At the end of that series of donut lifts is a gap in the ceiling. Wall jump up and you'll find a hidden pipe to the right. It leads to the second Star Coin. You will want the Ice Flower to attempt to get this Star Coin. It's in the path of the chains of three spike balls that swing together. Below is a pit of Bony Beetles. Drop down and freeze and destroy all Bony Beetles. Wall jump up the right side and get the Starman in the item block, then jump for the Star Coin. Return to the main level and use your invincibility to clear through more enemies. From the pipe to the main level, there is a spike ball swinging on the other side of a gap of donut lifts. Jump to the other side when the spike ball is swinging to the right and run past it. There's an item block with a power-up nearby. Dodge some more spike balls and Bony Beetles with good timing. Ahead are single donut lifts below a giant spike ball. You'll want to proceed when the spike ball is on the left. Move quickly. Ahead is another giant spike ball near a ? Switch. The ? Switch provides temporary platforms so you can cross the gap, and get the third Star Coin in the right corner, near the pipe. Timing is crucial here. If you can't quite get the timing right, use a P-Acorn. Drop down when the spike ball starts swinging to the right and boost repeatedly to the right wall, then boost to the Star Coin when the spike ball swings to the left. The pipe takes you to the flagpole. Thankfully, this castle has no boss to fight. There are donut lifts to the flagpole, but be sure to jump. Otherwise, when the donut lifts form stairs, you'll run right through them. This opens the path to a Mushroom House.

 Once you collect all of the Star Coins in Superstar Road, you unlock a bonus level, Superstar Road-9. You get a message: "The final course is now available! A fourth star has been added to your file!" And you thought you were done.