The guards are like freaking Oblivion pyschic. I've arranged for the executioner to get the real gun, but whenever he kills the tenor the guards go rambo on me no matter where I am. If I'm in the Light Room and no one has seen me enter, they storm the place so I don't have time to take out the ambassador. Does anyone know what I've done wrong?
I should first admit that I haven't played my PC version for some time, sticking mainly to the PS2 version, but as I remember they are identical.
Are you wearing a disguise? Most problems occur when you're wearing an inappropriate disguise for a particular area. What is your plan to get rid of target number 2? Sniper or bomb?
Whenever I have played this level by placing the real WW2 gun in the dressing room (in a workmans disguise obtained by pistol-whipping the workman in the toilet by the lobby), I usually place a bomb on the chandelier (up the back stairs where the workmen are hanging around)
Once I've changed back into 47s suit (important), then it's a matter of waiting in the main seating area (away from the crowd of tourists and the guards) until the crucial moment - the guards shouldn't notice you press the detonator when the bodyguards and the second target run down towards the stage (press it when the target falls over)
Your threat level will go into the yellow when you detonate the bomb, but if you walk quickly towards the exit while everyone is panicking, and avoid any contact with NPCs you shouldn't arouse any suspicion. I have always achieved silent assassin with this method.
I have tried many combinations of hiding in that lighting room, planting the gun, taking the shot with a sniper etc., and I've found it to be unreliable too, so I usually avoid it - I don't know why the guards seem to go straight there, but I have found that using the bomb on the chandelier, they usually go straight to the place where the bomb detonated leaving you a clear exit.
Hope this helps :-) --- "Only two things are infinite, the universe and human stupidity, and I'm not sure about the former." - Albert Einstein (1879-1955)