Step One: Farm Moonstones. You can do this in Serenity’s Waste, by farming Iwajira.
With a ~ Level 30 Hero you need about four Minutes per run and you get ~ 40 Moonstones per Run.
When you're, done just Quit the Game and then go to Continue. Pretty much do this till you have 400 Moonstones (40x per Moonstone Chest).
Or if you've been consistently collecting Moonstones throughout the game and have been opening Moonstone chests regularly, just keep playing normally and you'll eventually get this trophy halfway through your TVHM playthrough.
Step Two: Open Moonstone Chests. There are six Moonstone Chest Locations:
– Crisis Scar
– Pity’s Fall
– Titan Robot Embarkation
– Eye of Helios
– Vorago Solitude
– Moon Core
You can open each Chest multiple times. 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 until the trophy pops.
Wikia page for moonstoneshttp://borderlands.wikia.com/wiki/Moonstone