Requires the Delicious Last Course DLC.
Once you start up the game, you’ll be able to travel to Isle IV, the newest area. If you are yet to start the game, you can access the DLC after finding and beating the first Mausoleum.
Once you arrive and go through the cutscenes, you’ll be able to wander around the new area.
First things first, pull up your equip card and in the Charm slot, equip your new Astral Cookie.
This lets you play as Ms Chalice in a boss battle.
Then you can simply take the boat (found at the docks to the bottom left of where you start on Isle IV) back to Isle I, then battle the Root Pack (or any other boss you fancy) on Simple difficulty. Upon defeating the boss, this will unlock at the score screen.