This achievement requires that you kill 10,000 workers across all your plays of the game. Both Campaign and Survival mode kills count towards this achievement. You should save this one for last, as by the time you complete Campaign, you should be very close to if not beyond 10,000 kills. If you need to grind some kills, Level XI of Campaign is recommended. This mission can net you 450 kills in less than 10 minutes. You can check your progress towards 10,000 kills from the Main Menu by clicking the Settings button (looks like gears) and then Stats.