Tales of Vesperia: Definitive Edition

The fastest way to unlock this achievement is to buy Dark Bottles, use them and run around in a circle on the over world map defeating the weakest enemies you can access. Try to group them up to make it faster.

You can check your progress by hitting Y, going over to "Records" and scrolling down to "Total Enemies Defeated".

I usually unlock this achievement right after landing in Nordopolica in all the times I've played this game on X360, but it may come much sooner or much later depending on how many enemies you fight. If you play the game "normally", you are guaranteed to unlock this before the end of the game.

If for some unholy reason you do not have this by the end of your first playthrough, simply buy the "Records" option from the GRADE shop to transfer your record data, including enemies defeated.