I had every other achievement and was getting kind of bored hoping elites spawn while doing bounties, so I was looking for a better way, and I found one in a reddit thread
here so credit to reddit user "oddisy" for it.
Basically here's how it works.
1. Start up adventure mode normal difficulty in act 2.
2. Use the waypoint for archives of zultun kulle.
3. Go northeast a screen or 2 to find the teleport to the boss battle.
4. Command your skeletons to kill them all. At level 70 I can kill them all in 10-20 seconds. If you're properly geared it would probably be even faster.
5. Quit and choose continue to respawn kulle and slaughter his annoying ass again.
6. Repeat until you have 500.
I found I got achievement credit even if I didn't command the skeletons to attack, but if you have the frenzy rune they attack faster when you command them. Finally, make sure you don't have any thorns. Kulle especially LOVES attacking you, so thorns can potentially take the kill from your skeletons. You can use blood rush to get there a tiny bit faster if you like, too.