Once you have discovered the NES006 Specimen, gold bar combatants named [b]Assimilated Demolishers[/b] will begin to appear in Echoes Battleground activities. Destroying one of these enemies will result in a high chance of receiving an [b]Assimilated Beacon[/b] at the end of the activity. With the Beacon in hand, travel to the [b]Watcher’s Grave[/b] fast travel point on Nessus, follow the wall to your right in the direction of the Cabal Land Tank and you will find a special Vex Conflux. Using the Beacon on the Conflux will spawn three [b]Assimilated Demolishers[/b]; killing all three Demolishers (any other enemies can be ignored) within the time limit will award you with a [u]guaranteed[/u] 400g Radiolite Sample that you have not yet obtained. You can receive this guaranteed reward once per week. It’s highly recommended that you hit up a Public Event Rally Flag or bring some friends to ensure you have enough DPS to take out the targets before they despawn. (΄◉◞౪◟◉｀)

Once a week, so you have 3 opportunities left... Kill the world bosses in Watcher's Grave... if you don't want to grind for a beacon, you can just go hang out there and wait for someone else to spawn them.