You have limited time to get to the target location. Once there do not enter the base, dozens of armed guards (that look like normal pedestrians) will attack you upon sight. Once the courier arrives, it will go into one of the three nearby cars. Destroy the courier in the car to finish the mission. If a battle occurs when the courier arrives, he we flee. The best opportunity to kill him is when he makes his way to the cars near the entrance of the area.