Depending on which automation you use, you can add NPCs to a list that you pull within the radius you define. It'll stick to killing those and your guys will return to camp waiting for them to respawn. You can also exclude certain NPCs if there are more NPCs you want to kill than not. There'll be the chance you'll gain aggro from other NPCs and they'll get slain for getting in the way.
It's best to check against a website to ensure the npc is a placeholder for the zone you're in. The PH icon you see on the target window indicates that NPC is a placeholder. There is the possibility that the version of the one you're targeting isn't a placeholder, but same name exists in a different zone, where it is.