This one is can be boosted in any level with a vendor. Load up either MARKET, ARMORY or POLIS as these are the easiest to get to the vendor.
Now sell your weapon to him. After that back out of the screen and talk to him again, this time buy his cheapest available weapon. Now repeat this cycle until you unlock the achievement.
If you run out of MGR currency you can sell your ammo to the exchange kiosk to get some more or you can just reload checkpoint as this doesn't reset your progress. Make sure to reload checkpoint after you get your achievement so that you don't lose all your hard earned MGR.