After making your way up the stairs to the courtyard where Von Schrader is found, use Adrenaline mode and defeat him. Then, run out of the courtyard and go up the stairs that were previously barricaded. Proceed to take out the two guards, then run through the doors until you get to a large control panel overlooking the courtyard. Press X, and the last objective of "Eliminate Von Schrader and Disable the Dirty Bomb" will be completed and the mission will be over.