Postgame, you are able to find hidden areas around Peach's Castle, one for each world, by using the Dig ability received by completing the game. These areas contain secret, extremely difficult challenges, and beating them grants you a cash price and the ability to purchase ultimate weapons. There are eight ultimate weapons in total, one for each character, and it can be a primary or secondary weapon, depending on the character in question. Each of them are the highest damaging weapons of their kind and each of them ups damage by 10% against Midbosses.

Unlockable	How to Unlock
Luigi - Up Close & Personal (Precision)	Clear Ultimate Challenge 2, then purchase this primary weapon from the Battle HQ.
Mario - Grid Gavel (Melee)	Clear Ultimate Challenge 1, then purchase this secondary weapon from the Battle HQ.
Peach - Scubam! (Grenaduck)	Clear Ultimate Challenge 3, then purchase this secondary weapon from the Battle HQ.
Rabbid Luigi - Righteous Rocket (Rocket)	Clear Ultimate Challenge 1, then purchase this secondary weapon from the Battle HQ.
Rabbid Mario - Kite Commando (Boomshot)	Clear Ultimate Challenge 3, then purchase this primary weapon from the Battle HQ.
Rabbid Peach - Iron Curtain (Sentry)	Clear Ultimate Challenge 2, then purchase this secondary weapon from the Battle HQ.
Rabbid Yoshi - Kimono No No (Rumblebang)	Clear Ultimate Challenge 4, then purchase this primary weapon from the Battle HQ.
Yoshi - Sixty-Watt Swat (Rocket)	Clear Ultimate Challenge 4, then purchase this secondary weapon from the Battle HQ.