Step One: Farm Moonstones. I did this at Serenity’s Waste, farming Iwajira. With a ~ Level 30 Hero you need about four Minutes per run and you get ~ 40 Moonstones per Run. If your done just Quit the Game and Continue and do this till you have 400 Moonstones (40x per Moonstone Chest).
Step Two: Open Moonstone Chests. I know of six Moonstone Chest Locations:
– Crisis Scar
– Pity’s Fall
– Titan Robot Embarkation
– Eye of Helios
– Vorago Solitude
– Moon Core
but you can open one Chest again and again. The fastest Way is to run trough the Titan Robot Production Plant again and again, it will only take about two minutes per run. If you opened one Chests quit the Game and Continue till the Achievement pops.