So you probably already know that you can play as Nabbit in New Super Luigi U's multiplayer mode. But did you know you can play as him in Single-player too? Which is a huge help if you're having trouble, since Nabbit is invincible after all.
To do so, you simply need to hold down the correct button on the World Map as you select any level. But the exact button that you need to hold will depend on the controller you're using. If you're playing with a GamePad or Wii U Pro Controller, simply hold down the ZL-button as you select the level. But if you're using the Wii Remote, hold the B-Button instead.
In either case, you'll know if you did it correctly right away as Luigi's head will be replaced by Nabbit's on the level-loading screen.
Now that you know how to play as Nabbit, you can also play as your Mii as well using the same cheat. Your Mii is only playable on the one level that Nabbit is not, which is Super Star Road 9 Flying Squirrel Evasion, unlocked for finding every star coin from the first 81 levels. You can even see how you would look as Luigi once you reach the end of the level.