T_Dub

The best place to do it is Xi Laing and you need to head straight to the main camp when you start the mission but make sure that you kill any officers on the way or that are heading to your main camp. Your character will need to be pretty strong or you will need to play on an easy difficulty setting because the officers will respawn at the main camp and the soliders will keep coming no matter how many times you kill them. The reason for going there is it is a small map and there is only one way to each main camp. Once you get 500 kills just go take over the other small camps and head back to the main camp again and finish them off.