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.
*** Spoiler - click to reveal ***As soon as you initiate the fight it will immediately spawn 4 Slaves of Torment, 2 Cyclops BoneCrushers and 1 Cyclops Moondancer.
Each Slave of Torment will heal Balberith for 25% of his health every two turns and provide elemental immunity of the puddle they're standing in for him. They are weak against the element of their own puddle, immune to other elements but have no resistances to physical damage. They do not attack.
The bonecrushers can deal some serious physical damage and need to be crowd controlled as much as possible(charm arrows work well.
The moondancer poses little threat but will destroy summons that it can see.
Balberith will summon one additional enemy every two turns up to three total. He uses fear and has a pretty strong physical attack. Remember he will get healed from the Slaves of Torment.
The general strategy is to kill off the Slaves of Torment until there are either one or zero left and then kill Balberith while trying to deal with the adds. Once you kill Balberith you will need to finish off all the adds to complete the fight.