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Step 1: Gather 300-325 Moonstones

If you're starting this from scratch, you will ultimately need 400 moonstones to open 10 chests, but with the right chest farm you can go with 300-325 stones.

I recommend doing this on a higher level character that can kill Iwajira in Serenity's Waste on normal mode within seconds. Using this method to farm moonstones will result in 20-30 stones every few minutes.

Step 2: Farm Chests

Perhaps the fastest way to farm moonstone chests is through the Titan Robot Production Plant on normal mode. You will only need approximately 300-325 stones via this method, as killing the boss will net you around 10 new stones per run plus the chance of getting stones in the chest room.

Fast travel to the robot plant and run straight through to the end (it's a straight shot) where the boss spawns. Kill it, loot any stones it drops, and go to the room that opens in the back to open a moonstone chest. Open every chest in the room for a chance at more stones.

Quit, reload, and repeat until you have the achievement.