Level 16: Stop Wildfire
Mission 16 is best suited for multiplayer co-op mode, because the object is to assassinate three enemy engineers at the same time to prevent them from blowing up the bridge. But it's pretty easy to complete the mission successfully in single-player mode, as long as your commandos are positioned correctly.
The mission starts with your small group of commandos together on the eastern bank of the river. Your first task is to move the Spy southward to the clothesline to grab the German uniform. When the train goes by, kill that first guard from behind and hide his body behind the building. When that's done, crawl clockwise around the house and wait until the single patrolling guard begins his way west. Stealthily move southward -- avoiding all guards -- and hide in the house to the left. When the three patrolling guards head south, follow behind them and get the uniform off the rack. Your next step is to have the Sniper move southward, following the same path that the Spy took. It will actually be a bit easier to get him down to the bottom of the map, because you can use the Spy to distract the guards. Remember -- there's no reason to engage the enemy at this point. Keep distracting guards as the Sniper moves all the way south, and then all the way west, crossing the train tracks until he's safely nestled in his assassination spot on the southernmost edge of the eastern bank.
Once that's accomplished, this mission really is a piece of cake. Have the Marine hop into the water and swim to the island in the middle of the river; he should be positioned just to the north of the detonator. When he's in position, have the Spy run across the bridge, bang a left, and run down to the waters edge. You want to position him under the bridge, to the right of the detonator. You won't be able to see him, but that's okay, because from that spot he'll be able to kill the engineer and remain unseen by the other Germans.
Now that everyone's in position, use multiple cameras so you can see them all at once. Switch back to the Sniper: he's going to make the first kill. When the coast is clear have him assassinate the engineer just to the northeast of his position and then immediately go back into a crawl. When the engineer goes down, the one next to the Spy will run for his detonator. When he's within range, have the Spy shoot him with the pistol. He should be safely concealed under the bridge so no other guard will see the killing, but be careful just in case. That action in turn will alarm the two engineers on the island. Have the Marine spring from the water, and use the harpoon gun to kill the first one and the knife to kill the second, and then dive back into the water. If all went as planned, the engineers will be dead, the bridge will still be standing, and the guards won't have any idea what just happened.
From that point, the mission is a bit time consuming but by no means difficult. Have the Spy cross back over the bridge to the eastern side of the river, and position himself to the right of the escape truck. You want to distract the patrolling group of four guards so that they're all to the right of the truck, and facing northeast. Sneak the sniper past the patrolling guards, across the railroad tracks, behind the four guards that are being distracted by the Spy, and then a little farther east (in the middle of the road is good). From there, have the Sniper use his last four bullets to shoot the four distracted guards, and then immediately go prone. When they go down, another guard may come over to investigate; if that happens, use the Spy to kill him from behind. When the area to the right of the exit vehicle is clear, have the Sniper crawl over and wait there.
Your next step is to get the Marine to the same position. Have him swim northeast to the right bank and get out of the water at the exact same point he got into it. From there, have him travel all the way east and then all the way south, using the Spy to distract guards as needed. It should be a pretty easy to get the Marine safely to the right of the escape vehicle, with the Sniper. Once they're there, move the Spy over to join them and have him get in the truck. When the Sniper and Marine get in truck, it will drive away and the mission will end.