I got this in Phokis after leaving Megaris (start of Episode 3), because the first leader you kill in Megaris as part of the main storyline does not count towards this. All you need to do for this is avoid killing the Nation Leader until you have weakened the area. This means you have to kill soldiers and captains, burn stockpiles (purple marker) or raid their War Funds (purple chest). When the purple bar on the right side of your screen is low ie. Vulnerable and the conquest battle is available, you are safe to kill the Nation Leader for the achievement. If you want to check the purple bar for the area at any time just bring up the map and it should be on the right side of the screen.