The trophy description is pretty clear, so I'll just give some context. For me, this popped when I was grinding in Chapter 6. My party was around level fifty (some higher, some lower), I hadn't used enemy difficulty modifiers in my playthrough, and I'd done my grinding on normal-leveled enemies to that point (as in, I didn't go back to the early dungeons and rack up encounters to boost this).
Of course your mileage may vary, but I would be very surprised if you did not get this naturally by playing through the game. If you really want to, you can go back to the first dungeons and grind easy encounters for this trophy.
Interestingly enough, this trophy actually pops at the beginning of the battle. I found this ironic since the trophy seems to imply it will unlock upon the 500th victory.