This trick requires that you have the scope for your Silverballer. When the mission begins, run up the steps and to the right. Toss a bomb and quickly pick the lock on the first door on the left. Walk in and explode the bomb. Wait a moment, then snipe the man up in his private box and the actor walking on stage. Put your gun away and get out of there. If done correctly, you will get a "Silent Assassin" rank in under a minute.