Just as a sidenote when you look for the spy - protection paladins' shields light like christmas trees when near him since he's a demon. Demon hunters can see him with their spectral sight but it's not nearly as cool as the effect on prot palas.
In case anyone is still wondering the trigger points are.
1. Deactivate all 5 beacons before pulling the boss and make sure that no guard activates a beacon.
2. Identify the spy on the first try.
3. Loot the key and listen to the conversation (without opening the doors).
We got this by accident last night. Did #1 and #2. We opened the doors and walked in, listened to the conversation while on the balcony, one-shot boss, got achieve. Hope this helps.