The achievement states: "Kill an enemy hero with a Setseke." I first did this with auto-combat and a Setseke in my army but it didn't work so on the second try I used manual combat and made sure to get the killing blow with the Setseke. After passing the turn, however, the achievement did not appear which leads me to believe this achievement is bugged.

No mods were used and the game version number is 1.6.4 S3