This achievement is for defeating 500 normal J'avo. If you are checking the Dog Tag Titles then that number will be a combined count of all J'avo types but you have to kill 500 of the ones without weapons for it to count; check the Records section under Enemies for the accurate break-down.
This should come naturally if you play through a couple of campaigns that have regular J'avo in them, I ended up with around 375 after playing through just Chris' campaign. The first two of his chapters have many more enemies that will count towards this so just remember that if you die you keep your kill count and can gain more after resuming from the checkpoint to do it all again.
You can also play Mercenaries to increase your J'avo kills provided they carry no weapons. (UPDATE: When my achievement popped I had 472 J'avo and 30-40 in other subcategories that pushed it over 500, just not sure which one. Might be the rocket launcher since it doesn't seem to be counted as a weapon.)
Hope this helps, please leave comments for negative feedback and let me know what to improve for this guide.