The hardest part of trying to complete this achievement was fighting him indoors in the throne room. You can stay center to the horizontal red carpet, but you’re dependent on ranged actions if he ends up moving into a zone that triggers the trap bomb.
You may also have instances where a character is pushed into a trap zone, triggering the bomb. You can pull him out onto the balcony to make this an easy achievement.
Setup for Success
Preface that my playthrough included speaking to Gortash at Wyrm’s Rock Fortress and creating an alliance with him, then completing the Iron Throne and Steel Watch Foundry missions, making him hostile.
You can separate your party and leave three members outside on the balcony and pull the mob outdoors where there are no triggered traps related to the achievement. There are two normal traps you can disarm ahead of time near the doorway to the throne.
Have one person go in who has the Dimension Door ability/scroll, trigger the cutscene to begin the fight, and immediately use Dimension Door to teleport outside with the rest of your party. It will take a couple of turns, but Gortash and his mob will make their way outside.
You can setup traps at the doorway, such as wards and Cloud of Daggers, that they pass through for some initial damage.
In my playthrough, I had Karlach go in and trigger the fight (no real dialogue necessary between them) and teleported out.
Not sure if it was because of the distance from the initial start point and/or being outside, but Gortash’s reflection shell was gone so spellcasters could nuke and melee could easily get through his defense shield.