After completing Boethiah's daedric quest, you are rewarded with the Ebony Mail artifact. In addition to being to move more silently, the armor gives you a constant 5 point poison damage effect for enemies that get too close.

While traversing any dungeon, or any areas with doors for that matter, instead of sneaking around and silently taking out opponents, get their attention and run into any room that has a door. In their pursuit of you, your opponent will give chase all the way up to the door, and will remain on the other side.

It might take a minute or so, but the 5 points of poison damage will wear your enemy down through the door and kill them. They might open the door once or twice themselves, but just shut it again and stand close to the door.