Perfect Dark Zero

The little trick in my video should help you get 1000 headshot kills in no time at all. You should have a second controller for this. It isn't required, but is highly advised.

Match setup:
Team Killcount (Deathmatch)
Map: Desert, small variant
Custom weapon setup:
In slot 1 - 4 add flashbang or shield
In slot 5 add the Jackal
Bot setup:
Bot count: 14 (to allow your other player to join)
Difficulty: Agent
Bot teams: Humans vs Bots
Personality: Bully

Set the time / score limit to whatever suits you, just make sure you don't set the time limit to infinite (the game won't be able to end properly and the kills won't count toward the achievement.) 20 minutes with no score limit will get you close to 100 kills.

A much faster and easier way to do ANY of the 1000 kill achievements (with the exception of 1000 kills in killcount, and possibly 1000 explosive kills - the guide for explosives appears to work faster than this solution) is to set up a local match on TEAM killcount. Plug in as many controllers as you have laying around (guitar/drumset controllers will work for this too, though you can't move the characters with them) and set up a match on the small variant of the desert map. Set up the teams so that it is you vs. everyone else.

Now, depending on what achievement you are going for, create a custom weapon setup with a weapon of that class in the first slot so everyone spawns with them. For example, you can place the jackal in the first slot if you're going for the "x Sniper Kills" achievements, or a rocket launcher if you're going for the "x Explosive Kills" achievements. This will ensure that you'll never have a problem finding ammo for said weapon since whenever you kill someone, they'll drop theirs.

You should be good to go now. All you need to do now is set a round limit. I prefer 20 minutes or 100 kills, whichever comes first. When the round starts, all you need to do is run to the area in which they spawn and kill the non-controlled characters as they spawn. They should always spawn in the same area, so just pick them off as they pop up. I've been using only one extra controller, and I've been getting 100 kills within 20 minutes every single time. It's worth noting that I run the 2nd controller into the teleporter myself rather than waiting the 15 seconds for it to go automatically. Otherwise, you'll end up with less kills for the round.

For this specific achievement, just be sure to kill them with headshots. If you're using a sniper rifle for this, you can knock out both achievements in one go.

Hopefully this helps you as much as it's helped me.

Here's a video I created back in Oct 2006 of how I did this achievement.

There may be quicker ways, especially if you're a great shot.

Good luck.