Gortash Achievement

[Carl Wheezer]

Hello all, I've tried getting this achievement numerous times and have turned here for help. For context I'm doing it after the coronation. I go up the side of the castle into the room behind the throne and stay in that room and pull in all the mobs from the throne room. I kill them one by one to make certain I don't activate any traps. I then run through the throne room (not in stealth now that i think about it but had no indicator showing the traps had activated and took no damage) to the door on the left and up the staircase to the rooftop. I then run along the edge of the rooftop avoiding the stun traps and leave three party members outside and have astarion enter dialog with gortash in his room. Astarion then runs and blinks out of the room back onto the rooftop where I pull gortash and kill him. I never see a grenade or flame during the whole process but I simply can't get the achievement to trigger. Is there something I'm doing wrong?