If you use a weapon at least once you are good to get this achievement, but you will also need to buy some weapons. You can find the Helsing (rare to find) at the Front Line level on some sandbags (after you climb up from an abandoned area) and you can find a Tihar in many levels. To ensure you get this early, always take a gun you never used from the ground and use it at least once to kill something. I got this achievement in the level after Pollis, where I got the last weapon needed: Kalash 2012.
