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.
It won't matter if you obtain the same weapon a second or a third time or whatever.
So it's pretty straightforward: when you can spend around 80k gold, just go to the first town that sells weapons and then keep buying the first weapon available for 800 gold until you hit 100 and then the trophy unlocks.