The Final Battle (New Super Luigi U) is the second castle and the sixth and final level in Peach's Castle in New Super Mario Bros U. This is the only level in the game that shares the same name with its New Super Mario Bros. U counterpart.
The player starts on a platform as their character watches Bowser Jr. slam down on a bone platform in his Junior Clown Car, and he continuously does this throughout the level. It sinks, then the level starts. Two Magmaws are in the lava. A Red Ring appears, and after three bone platforms, the first Star Coin appears. Multiple bone platforms appear after that, along with two Magmaws. A row of ? Blocks is found, followed by another row of bone platforms. Then, a door appears. This leads to Bowser's door. Entering Bowser's door gives the players an extra 250 seconds.
The player must work their way through the hall. They encounter Bowser. They must hit the ! Switch on the other side of Bowser, causing him to fall. The player progresses in saving Princess Peach. Meanwhile, Kamek (assisted by Bowser Jr.) puts a spell on Bowser, which makes him huge. The player character makes their way up stairs, and notices Princess Peach in a tower. She calls for the player character(s), but bars trap her, along with a wall covering the window with the Bowser Jr. logo. Bowser Jr. comes up, laughs at the character(s), then flies up to allow the gigantic Bowser to hop up.
To damage him, the player must wait for Bowser Jr. to slam down in his Junior Clown Car. They must then jump on him, knocking him out. Then they must hop in and the player must press the ZR button to go higher. Then, when at Bowser's head, the player must press the B button to slam down. Bowser will then go into his shell, and slide about, occasionally going higher, so the player can run under him. Bowser will soon pop out, breathing fire in the air. The player must complete this action three times to defeat Bowser.
- Star Coin 1: Below some Brick Blocks, the player must use Bowser Jr. to slam down and break them. Then, the player must Wall Jump to grab the Star Coin and avoid falling in the lava.
- Star Coin 2: The second Star Coin is located below several ? Blocks containing 1-Up Mushrooms. Use Bowser Jr. to slam down and destroy some, accessing the Star Coin.
- Star Coin 3: Near the door which leads to the Bowser door, the player must wait on the third bone platform. As it goes down, the player will get the third Star Coin.