After you've completed all requests by Splotch and the crystals create the Golden Key, you'll be returned to the VR Menu. Unlocking the middle option with the key, Splotch will reveal Free Paint mode. Return to the Crystal Meadow and you'll actually have all items available to use now. This includes the last item, the Crystals. Just choose that and paint across the landscape until the trophy pops (I'd say about 15-30 crystals will pop it).