The Final Battle (New Super Mario Bros U) is the second castle and the sixth and final level in Peach's Castle in New Super Mario Bros U.
The first part of the course is a short section of lava and Bony Platforms. Bowser Jr. chases the player in his Junior Clown Car, periodically attempting to drop on their head and also may try to push the Mario Bros. and Toads into the lava below if they get too close. The player can dodge the attack by moving out of the way as Bowser Jr. drops, but he destroys any Brick Blocks he hits on the way down. The player cannot harm Bowser Jr. in this section (due to a glass dome over the cockpit of his Junior Clown Car), so they must simply avoid his attacks as they work their way through the course. Throughout the area, Podoboos are found between platforms, and Bony Beetles can be found on some platforms. A Warp Door is found at the end of the area, leading to the second area of the level. To the right of the door are two Hidden Blocks. The one on the top contains a 1-Up Mushroom.
The second area of the level begins with a Checkpoint Flag in front of a large green door. Two Hidden Blocks on both sides contain a Fire Flower and an Ice Flower. After navigating a corridor filled with incoming fireballs, the player reaches the first Bowser battle, which greatly resembles the first part of New Super Mario Bros. Wii's final boss. Hitting the switch at the other side of the course causes an Axe to cut the bridge Bowser is standing on, making him crash into the pit below. After the player moves on to the final part, Bowser Jr. arrives with Kamek, who casts a spell to enlarge Bowser.
The final part of the course takes the player to a courtyard overlooked by a tower holding Princess Peach captive. Before the player can reach the princess, they must survive a boss battle with Giant Bowser. Bowser Jr. appears and taunts the player with Bowser following. He then stands up and roars at the player, signaling the start of the true final battle. Unlike the three previous encounters, the player must directly harm Bowser by stealing Bowser Jr.'s Junior Clown Car and crashing it down on Bowser's head. However, Bowser Jr. cannot be defeated, no matter how many stomps or attacks from the player(s) he has taken, although he can be stunned. The player can then throw Bowser Jr. at Bowser to damage him, although this method is time consuming. The player can also shoot a large amount of fireballs at Bowser or perform Synchro Ground Pounds (with multiple players) to defeat him. While doing this, they must evade Bowser's enormous fireballs and spinning shell, as well as Bowser Jr.'s diving attacks and dropped Bob-ombs. After three strikes to the head, Bowser is defeated and the game is beaten. Humiliated, Bowser Jr. jumps down to see his father.
- Star Coin 1: The first Star Coin is located above a pit of lava, and the player must Wall Jump to collect it safely.
- Star Coin 2: The second Star Coin is found beneath a row of Brick Blocks, and the player must lure Bowser Jr. to attack above the Brick Blocks in order to destroy them.
- Star Coin 3: Similarly to the first Star Coin, the third Star Coin is found above a pit of lava, next to a moving platform. The player must Wall Jump to collect the Star Coin.