The code is different per person, please do not try the code provided below. Chances are minimal it will work for you. But included is HOW you solve the code since everyone has a different one. これがお役に立てば幸いです!
Before you can fight the boss you will require 2 もの.
1. Access to the graveyard
2. The new Relic
At the top right part of the map, beside the shop. がある 3 NPC’s on a podium. These npc’s will give you the code for the graveyard boss. When talking to them, take note of what they are saying because it wont be direct. Try and keep a eye out for key things like, 左, 上, 下. Depending on what position the npc is in when you talk to him (1st place podium, 2nd place podium and 3rd place podium) that will be the order of your code.
Talking to first place told me this. Going from the middle unusable tombstone, my first tombestone i want to hit is #7 (if you are looking at the tombstones like your keypad)
The second one told me “all right”. Which is #6, and my 2nd digit to the code.
The final one mentions “down left”. これは #1.
So my code for my game is 6, 7, 1.
*ご注意ください, once you kill the boss, you are unable to re-challenge him
After killing the boss, go back in to your inventory and look at your relic
Your broken relic has been cursed by the boss you just defeated and now has a real use! 楽しむ! 
This Item will limit you to 1HP at the start, but give you every weapon for the battle, along with recovering HP through parry’s like the Heat Ring
You will also receive this achievement