not really sure there is a specific level. one play through should net you 500-600 kills. And while i was trying to get the 100% accuracy achievement the 750 kill one popped. another thing to note is if you get to a spot but dont complete the objective, guys will just keep spawning.
like in the sniper mission, when they want you to snipe the cannons up on the hillside. if you dont snipe those guys, people will just keep spawning and you can easily get alot there.