Divinity: Original Sin - Enhanced Edition

For this achievement you will need to have Jahan in your party and kill Balberith whom is in the Phantom Forest at coordinates x:394 y:259 down in the basement. This is a difficult fight because Balberith is level 19 and there will be a bunch of adds to deal with. The achievement will pop as soon as Balberith dies even if the fight isn't over yet.

Edit: According to the comments, you may not need to have Jahan in your party to unlock the achievement. Do so at your own risk, but it certainly adds more options for you to tackle this fight.

Click for fight details and a general strategy. Keep in mind that the strategy provided will be very basic since parties in this game can greatly vary.