i confirmed my theory on this one.. if you noticed, there is a buff on those npc that makes them invulnerable.. what you need to do is to wait for the buff to expire then talk to them.. you need to keep them alive or the achievement will fail.. i talked to 7 npcs since i soloed doing this achievement.. all 7 npcs didnt die.. and i still have 2 mins on the 10 minute countdown.. i hope it helps.. good luck!