Cuphead The Delicious Last Course DLC How To Unlock The Secret Boss (Graveyard Puzzle Solution)
The first thing you want to do is go to the shop and buy the Broken Relic. This will cost you 1 coin. If you don't have a single coin you can go behind the shop and walk left to go behind the buildings where you will find a coin at the end.
Now equip the relic and talk to the 3 climbers from 1st place to 3rd. Each one of these NPCs will mention a direction when speaking (this is random for each player, in my case it was UP, LEFTDOWN & RIGHT).
Finally go to the graveyard and press A on the gravestones in the order that the climbers mentioned (always starting from the middle gravestone e.g. in my case UP = the top stone, LEFT DOWN = the bottom left & RIGHT = the stone to the right of the center). If done correctly a beam of light should shine down on the middle gravestone allowing you to fight the secret boss.