I found that if you go through the Zhent back entrance and go down the lift. You could kill one of them completely in stealth. Throw a fireball and go back into stealth, it will just look dumbfounded for a moment and resume it's patrol. My first time I came through it, I was spotted and didn't know what spotted me until one of them leaped and ground pound me into sausage meat, killed 3 of my characters with one hit. I was level 4.
