You earn this achievement by killing a Roving Clans city while it has the Setseke Ho! ability activated. The problem is, in 80 hours of gameplay, I've witnessed the AI use Setseke Ho! all of 0 times even as I closed in on their last city with my armies. What I did was simply enlist of the help of a friend in a multiplayer match. You could do it yourself if you have two computers able to run Endless Legend.
Select a small map size and "close" Empire spread so that you spawn near each other and have your friend settle on the first turn. The next turn, have them activate Setseke Ho! while you make your way to their settlement with your starting army. Make sure to allocate your workers into influence - the purple star icon - so you have the influence points to declare war by the time you reach them. When you do reach them, declare war then attack the city and you should win handily, unlocking the achievement. You should also unlock the following achievements with this method, if you have not already: