I would highly suggest to only buy 1 health potion from the Leprechaun, otherwise you make it a lot harder for yourself on the puzzle with the graveyard key.
The developer says on steam forums that there are enough rubies in the game, but I found a few more in the last dungeon and you can already spend too much on the health potions before you're able to unlock it. So don't make this harder for yourself, only buy 1 health potion and you are able to get this achievement and still have enough rubies for that specific puzzle.