You need 75 gems to unlock this level. I have 105.
Toadette finds herself on a tower with only a clear pipe. Launch to another tower with Fire Bros and shields that are touchstones. The plants contain turnips you can throw at the lower Fire Bros. Hit the one not on the stairs and walk up the nearby ladder. Step on the P-Switch to reveal a clear pipe near where the Fire Bro was. Rotate the camera around to the back so you can see where the first gem is in the clear pipe. Exit the pipe at the top of the stairs and you'll stomp a Fire Bro. Go down the stairs to stomp the rest. Once you defeat all of them, a bridge will drop so you can cross to the tower with the clear pipe. Be sure to collect as many coins as you can while you're going through the course and try not to die.
The clear pipe cannon will launch you to a tower with Magikoopas. There's a turnip plant in the center that will give you unlimited turnips. Toss them into the pipes at the lowest level and they'll cross to the other side to hit the Magikoopas. For example, if a Magikoopa is on the left side of the tower, in front of the right clear pipe on the left side of the tower, throw the turnip into the lower left pipe. Clear the two Magikoopas out and the bridge to the next clear pipe cannon will drop. You'll land in an area with the Toad Brigade. Approach Blue Toad for a Mushroom. There's also a ladder that leads below the floor, where the second gem is.
Launch from the clear pipe cannon and climb the tall ladder. You'll see Captain Toad and the Star, as well as Goomba Stacks and Hammer Bros. Carefully make your way past them towards the center while avoiding the purple water, where there's a handle. Pull it and it becomes a turnip cannon. Use the turnips to defeat all enemies and clear out all of the bricks. You'll find the third gem in one of the corners. Be sure to defeat the enemies on Captain Toad's side. Once all of the enemies have been defeated, the bridge will drop. Grab the Star and Wingo will pick up Toadette with his beak and blow Captain Toad away by flapping his wings. He falls off the tower. This concludes Episode 2 and Episode 3 is now open.
Special Challenge: Collect 170 coins.
You'll need to collect the vast majority of coins on this level without dying. If you do die, there's a checkpoint when you reach the Toad Brigade, so you'll have to start over.