in Living Tombs you go under the aquaduct to the next area where the second priest is for the Silent City Access Quest, go to the climbable wall that is in the far left area and leads up to the giant Spectral Hounds but do not go up it. There is a small stair incline leading up the curb right before it, enemies cannot go up this slight step. To exploit this simply attack any enemy and when they are close to you simply walk up or down the stairs away from them, they will walk 20 feet down the curb and twenty feet back to get to you, and when they do you can simply traverse the stairs again, making it possible for ranged characters and mages to easily dispatch any enemy in the area solo. Essentially the stairs are an invisible line that the enemy cannot cross, instead they walk a large distance to the next crossing.
There is a similar bug in Everfrost; go through the canyon with the small wolves in it, and right when you get to the mouth as it opens up there will be a small incline to your left, this is another of those "invisible walls" that monsters will not cross. Take your pick of any monster in the area and whenever you need a break simply step over the incline, the monster will travel very far around the large rock instead of going down the tiny slope. If you are a pet class it works well to simply have your pet attack the monster while you go back and forth across this line, if not, simply use plenty of DoT's and use it as a personal immunity.
As a side note, these bugs work on multiple enemies, meaning you can pull the whole zone and none of them will cross that line. The fun bug is simply that if you can get on the wall in SQ using the house by the Arena, if you get behind the Arena there is a door that the creators forgot to remove.