When the mission begins you have to down a guard on a sniping post. Crouch or walk quietly to sneak to him. Get close and attack him with your knife, then shoot some guards before you regroup with your units. Shoot only when you hear thunder. You can predict when it happens by noticing the lightning. Take out the one on the left pier first when thunder strikes, then aim to the right where you can see two other guards on two piers. The thunder will strike, and you can take them both out quickly. You can also wait for thunder to strike again and take them out individually. Reload and aim up slightly. When aperson comes down the stairs, hit him when thunder strikes. You must now take out two guards at the same time with a one shot kill when thunder strikes. This can be tough because the alarm is sounded when gunshots are heard.