This achievement will come naturally as you play through the game. There are plenty of monsters in the wild to kill and you will be fighting them either as you explore or as parts of main and side quests. I got this achievement at level 29.
If you want to speed up the grind with this achievement, you can use the Coin Arena. You can replay any previous challenges that you have completed or reached. If you'd like to grind monster kills then play some of the earlier challenges that have 5-6 monsters and aren't too difficult. You and your companions should be able to take them out quickly and you can keep replaying the challenge until you've reached 600.