You will get this naturally as you progress through the game, there are way more than 500 enemies to kill in a full playthrough and if you are going for 100% you must kill more than 500 enemies to complete the Betiary for the other achievements so I'd suggest just playing rather than concentrating on this.
However, if you did choose to grind this out as fast as possible a decent time to do it is right after you escape from Nalbina Dungeons and are in Dalmasca Sands. There are lots of Wolves and Cockatrice's which you and your party can simply 1 shot by this point. Set gambits on each of your characters to attack nearest enemy, including the party leader, and just run through the area. You will auto-attack the enemies and 1-shot them. It's a decent enough time to grind up because you can get quite a bit more powerful than you need to be, making the rest of the game easier.
You can easily link this together with other achievements:
^ These you will also get these naturally so not necessary to grind it, but feel free to do so
^ Set your gambit to cast a spell rather than attack. To have spells at this point you need to be a white/black/red mage for this
^ You start with steal, so you steal from the enemies while your allies attack them.