I tried a number of times with an invisibility scroll, but never got a favourable configuration of the guards. I eventually went away until I was stronger, and then came back and depopulated the place. My patience didn't last as along as yours!
I think that's the only way I could do it now – my scores on "Minesweeper" indicate that my digital dexterity has dropped over the years . I want to have another go at the old "Eye of the Beholder" games sometime to see if I can still play them adequately. There's no turn-based option in those . . .