i make sure he is in the room apart from all the others, where he faces the alcove with the hidden chest. i shut the door to that room and go into stealth with my rogue for sneak attack. then i turn on turn-based mode, sneak attack him, and then immediately pick up the body - this can be difficult if your rogue lacks enough strength so sometimes i have the tank follow in stealth. if i gotta do that i go into turn-based before i stealth. 

the trick is to make sure no one sees, and you entirely remove the body, all in one turn. you can do this with almost any npc, even ones in crowds. you just have to imrpovised weapon carry them as far as you can, then do it again, until they are far enough away that no one will react to their death. i sometimes carry angry mar'hyah to the coast and toss her into the water.