Aside from the starting weapons (the ones that are already available as soon as you start Libra of Soul), every weapon counts, both earned and bought.
If you start doing secondaries, eventually you'll get the trophy naturally.
It won't matter if you obtain the same weapon a second or a third time or whatever.
Note that this same method applies towards the True Edge Master Trophy.
So it's pretty straightforward: when you can spend around 40k gold, just go to the first town that sells weapons and then keep buying the first weapon available for 800 gold until you hit 50 and then the trophy unlocks.