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.