Here is what I like to do whenever I visit karazhan. I did not see this posted here before so here is my contribution.
Start out at this point on the wall behind the kara summoning stone.
Wall jump at a slight angle so that you will reach the point where you can turn around and jump on the roof.
And from here you have a good angle to snipe the enemy from. If you need all you have to do is back away from the edge and you are out of line of sight from most people on the ground.
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