Quickest way I've found to complete this is to buy the Ashen Key mission(s) for what ever area your in (Shores of Plenty, Ancient Isles or Wilds) get as many keys as possible from those missions and sometime another Key Master will appear whilst your doing the mission.
Once you have as many keys as you feel happy with then head back to the nearest Outpost and purchase the Ashen Chest mission from Sticker Jim, not the 0 doubloon one but the 30 doubloon mission, that will give you a Ashen chest mission on the Outpost your on.
Once your located it and dug it up, open it and cash in the tomb and the chest and hope you get the one you need. Then repeat the process until you have them all.
Good luck. 👍