Pit People

You will get this achievement as a part of the story missions. The first mission after all of the tutorials is to attack Helmitor's fortress, which you can do once you have 6 party members. It is a simple enough mission, with you attacking his fortress by trying to get to Helmitor by breaking down the fortress gates. Throughout the first part of the mission, you will be under attack by an enemy in a mortar thing, which fires an explosive attack at the end of your turn. Also, there are four enemy tents throughout the inside and outside of the fortress which will spawn more enemies every few turns if they are not destroyed. Once the second gate is destroyed, the second part of the mission starts, where you simply need to kill Helmitor and his remaining minions with no gimmicks. After you finish the first part of the mission, the achievement will pop.

Also, if you're like me and finished this mission before the achievement was added, you can either start a new file and skip the tutorial, which will put you right at where you need to be to start the mission, or you can finish the story, after which point the story quests will reset, allowing you to do the Helmitor mission (as well as all of the other ones) again for the achievement.