Complete Stage 3 Day 1 in order to receive the Ancient Buckle which enables Arma-mode, a form in which Dillon becomes Invincible and deals triple the amount of damage to his enemies.
The Old Mission and Sal's Hut
After you clear the game's final stage, you'll open 2 more locations on your map: The Old Mission and Sal's Hut. The Old Mission is an extra boss that you can fight once you have at least $100,000 in your savings. Sal's Hut allows you to see 3D models of all of the Grocks in the game, you can unlock it by earning all 50 Stars from the stages.
The Old Mission Map Location
Complete the game with at least $100,000 in savings to unlock The Old Mission map location where you will be able to fight a hidden Boss.