Ok, so this challange is totally bugged for me...
1.) Once i'm spotted, certain NPCs tend to jump to their deaths (2 of the 3 balcony guards did that once)
2.) Hitting a weakpoint on Arbalist sometimes causes him to jump across the gap and no longer being stunned
3.) Once killing the melee guard next to the cage, the ranger comes down and jumps around and eventualy falls through the cage somewhere below the map
4.) Once killing all NPCs, I get 1 or 2 additional target somewhere outside the map???
5.) I don't know if this is intentional or not, but the weakpoints seem to shift around during each run, meaning it's sometimes impossible to position myself to hit a guard with sneak attack
EDIT: Is this a known issue? I did complete the challange once before, but now im going for gold medal, and its impossible to finish...